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Abstract:

It is an object to provide a semiconductor device typified by a display
device having a favorable display quality, in which parasitic resistance
generated in a connection portion between a semiconductor layer and an
electrode is suppressed and an adverse effect such as voltage drop, a
defect in signal wiring to a pixel, a defect in grayscale, and the like
due to wiring resistance are prevented. In order to achieve the above
object, a semiconductor device according to the present invention may
have a structure where a wiring with low resistance is connected to a
thin film transistor in which a source electrode and a drain electrode
that include metal with high oxygen affinity are connected to an oxide
semiconductor layer with a suppressed impurity concentration. In
addition, the thin film transistor including the oxide semiconductor may
be surrounded by insulating films to be sealed.

Claims:

1. (canceled)

2. A terminal portion comprising: a conductive layer; a first insulating
layer over the conductive layer, the first insulating layer comprising
silicon and nitrogen; a second insulating layer over the first insulating
layer, the second insulating layer comprising silicon and oxygen; a third
insulating layer over the second insulating layer, the third insulating
layer comprising silicon and oxygen; and a transparent conductive layer
over the third insulating layer; wherein the conductive layer is
electrically connected to the transparent conductive layer, and wherein
the conductive layer comprises a same metal element as a metal element
comprised in a gate electrode of a transistor.

3. The terminal portion according to claim 2, further comprising a fourth
insulating layer over the third insulating layer, the fourth insulating
layer comprising silicon and nitrogen.

4. The terminal portion according to claim 2, wherein the second
insulating layer and the third insulating layer are in contact with each
other.

7. The terminal portion according to claim 2, wherein the gate electrode
comprises: a first layer comprising copper, and a second layer over the
first layer, the second layer comprising a metal element having a higher
melting point than copper.

8. A terminal portion comprising: a first insulating layer, the first
insulating layer comprising silicon and nitrogen; a second insulating
layer over the first insulating layer, the second insulating layer
comprising silicon and oxygen; a conductive layer over the second
insulating layer; a third insulating layer over the conductive layer, the
third insulating layer comprising silicon and oxygen; and a transparent
conductive layer over the third insulating layer; wherein the conductive
layer is electrically connected to the transparent conductive layer, and
wherein the conductive layer comprises a same metal element as a metal
element comprised in source and drain electrodes of a transistor.

9. The terminal portion according to claim 8, further comprising a fourth
insulating layer over the third insulating layer, the fourth insulating
layer comprising silicon and nitrogen.

10. The terminal portion according to claim 8, wherein the second
insulating layer and the third insulating layer are in contact with each
other.

12. The terminal portion according to claim 8, wherein each of the source
and drain electrodes comprises copper.

13. The terminal portion according to claim 8, wherein each of the source
and drain electrodes comprises: a first layer comprising copper, and a
second layer over the first layer, the second layer comprising a metal
element having a higher melting point than copper.

14. A semiconductor device comprising: a terminal portion, wherein the
terminal portion comprises: a conductive layer; a first insulating layer
over the conductive layer, the first insulating layer comprising silicon
and nitrogen; a second insulating layer over the first insulating layer,
the second insulating layer comprising silicon and oxygen; a third
insulating layer over the second insulating layer, the third insulating
layer comprising silicon and oxygen; and a transparent conductive layer
over the third insulating layer, wherein the conductive layer is
electrically connected to the transparent conductive layer; a transistor,
wherein the transistor comprises: a gate electrode; a fourth insulating
layer over the gate electrode, the fourth insulating layer comprising
silicon and nitrogen; a fifth insulating layer over the fourth insulating
layer, the fifth insulating layer comprising silicon and oxygen; an oxide
semiconductor layer over the fifth insulating layer; a source electrode
electrically connected to the oxide semiconductor layer; a drain
electrode electrically connected to the oxide semiconductor layer; and a
sixth insulating layer over the oxide semiconductor layer, the sixth
insulating layer comprising silicon and oxygen, wherein the conductive
layer comprises a same metal element as a metal element comprised in the
gate electrode, and wherein each of the source electrode and the drain
electrode comprises a region, the region comprising a titanium oxide
having oxygen vacancy.

15. The semiconductor device according to claim 14, further comprising a
seventh insulating layer over the third insulating layer, the seventh
insulating layer comprising silicon and nitrogen.

16. The semiconductor device according to claim 14, wherein the second
insulating layer and the third insulating layer are in contact with each
other.

17. The semiconductor device according to claim 14, wherein a thickness
of the region is greater than or equal to 0.1 nm and less than or equal
to 10 nm.

18. The semiconductor device according to claim 14, wherein the titanium
oxide is a titanium dioxide.

21. The semiconductor device according to claim 14, wherein the gate
electrode comprises: a first layer comprising copper, and a second layer
over the first layer, the second layer comprising a metal element having
a higher melting point than copper.

22. A semiconductor device comprising: a terminal portion, wherein the
terminal portion comprises: a first insulating layer, the first
insulating layer comprising silicon and nitrogen; a second insulating
layer over the first insulating layer, the second insulating layer
comprising silicon and oxygen; a conductive layer over the second
insulating layer; a third insulating layer over the conductive layer, the
third insulating layer comprising silicon and oxygen; and a transparent
conductive layer over the third insulating layer, wherein the is
electrically connected to the transparent conductive layer; a transistor,
wherein the transistor comprises: a fourth insulating layer, the fourth
insulating layer comprising silicon and nitrogen; a fifth insulating
layer over the fourth insulating layer, the fifth insulating layer
comprising silicon and oxygen; an oxide semiconductor layer over the
fifth insulating layer; a source electrode electrically connected to the
oxide semiconductor layer; a drain electrode electrically connected to
the oxide semiconductor layer; and a sixth insulating layer over the
oxide semiconductor layer, the sixth insulating layer comprising silicon
and oxygen, wherein the conductive layer comprises a same metal element
as a metal element comprised in the source and drain electrodes, and
wherein each of the source electrode and the drain electrode comprises a
region, the region comprising a titanium oxide having oxygen vacancy.

23. The semiconductor device according to claim 22, further comprising a
seventh insulating layer over the third insulating layer, the seventh
insulating layer comprising silicon and nitrogen.

24. The semiconductor device according to claim 22, wherein the second
insulating layer and the third insulating layer are in contact with each
other.

25. The semiconductor device according to claim 22, wherein a thickness
of the region is greater than or equal to 0.1 nm and less than or equal
to 10 nm.

26. The semiconductor device according to claim 22, wherein the titanium
oxide is a titanium dioxide.

28. The semiconductor device according to claim 22, wherein each of the
source electrode and the drain electrode comprises copper.

29. The semiconductor device according to claim 22, wherein each of the
source electrode and the drain electrode comprises: a first layer
comprising copper, and a second layer over the first layer, the second
layer comprising a metal element having a higher melting point than
copper.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. application Ser. No.
12/898,366, filed Oct. 5, 2010, now allowed, which claims the benefit of
a foreign priority application filed in Japan as Serial No. 2009-235792
on Oct. 9, 2009, both of which are incorporated by reference.

TECHNICAL FIELD

[0002] The present invention relates to a semiconductor device that
includes a thin film transistor (hereinafter also referred to as a TFT).

[0003] In this specification, a semiconductor device generally means a
device which can function by utilizing semiconductor characteristics, and
an electro-optical device, a semiconductor circuit, and an electronic
appliance are all semiconductor devices.

BACKGROUND ART

[0004] In recent years, a technique for forming a thin film transistor
(also referred to as a TFT) by using a semiconductor thin film (having a
thickness of approximately several nanometers to several hundreds of
nanometers) formed over a substrate having an insulating surface has
attracted attention. Thin film transistors are applied to a wide range of
electronic devices such as ICs or electro-optical devices, and prompt
development of thin film transistors that are to be used as switching
elements in image display devices, in particular, is being pushed.

[0005] A thin film transistor is manufactured mainly by using a
semiconductor material such as amorphous silicon or polycrystalline
silicon. TFTs using amorphous silicon have low field effect mobility but
can be formed over a large glass substrate. On the other hand, TFTs using
polycrystalline silicon have high field effect mobility, but require a
crystallization step such as laser annealing and are not always suitable
for being formed over a large glass substrate.

[0006] Thus, a technique in which a TFT is formed using an oxide
semiconductor as a semiconductor material and applied to an electronic
device or an optical device has attracted attention. For example, Patent
Documents 1 and 2 each disclose a technique in which a TFT is formed
using zinc oxide or an In--Ga--Zn--O-based oxide semiconductor as a
semiconductor material and used as a switching element or the like in an
image display device.

[0007] A TFT in which a channel formation region (also referred to as a
channel region) is provided in an oxide semiconductor can have higher
field effect mobility than a TFT using amorphous silicon. An oxide
semiconductor film can be formed at a temperature of 300° C. or
lower by a sputtering method or the like, and a manufacturing process
thereof is simpler than that of a TFT using polycrystalline silicon.

[0008] TFTs which are formed using such an oxide semiconductor over a
glass substrate, a plastic substrate, or the like are expected to be
applied to display devices such as a liquid crystal display, an
electroluminescent display (also referred to as an EL display), and
electronic paper.

[0009] Moreover, there is a trend in an active matrix semiconductor device
typified by a liquid crystal display device towards a larger screen,
e.g., a 60-inch diagonal screen, and further, the development of an
active matrix semiconductor device is aimed even at a screen size of a
diagonal of 120 inches or more. In addition, a trend in resolution of a
screen is toward higher definition, e.g., high-definition (HD) image
quality (1366×768) or full high-definition (FHD) image quality
(1920×1080), and prompt development of a so-called 4K Digital
Cinema display device, which has a resolution of 3840×2048 or
4096×2180, is also pushed.

[0010] As a display device has a higher definition, the number of pixels
needed for it is significantly increased. As a result, writing time for
one pixel is shortened, and thus a thin film transistor is required to
have high speed operation characteristics, large on current, and the
like. In the meantime, a problem of energy depletion in recent years has
caused demand for a display device whose power consumption is suppressed.
Therefore, a thin film transistor is also required to have low off
current and suppressed unnecessary leakage current.

[0011] Increase in screen size or definition tends to increase wiring
resistance in a display portion. Increase in wiring resistance causes
delay of signal transmission to an end portion of a signal line, drop in
voltage of a power supply line, or the like. As a result, deterioration
of display quality, such as display unevenness or a defect in grayscale,
or increase in power consumption is caused.

[0012] In order to suppress increase in wiring resistance, a technique of
forming a low-resistance wiring layer with the use of copper (Cu) is
considered (e.g., see Patent Documents 3 and 4).

[0017] Since parasitic resistance generated between a source electrode and
a semiconductor layer and between a drain electrode and the semiconductor
layer in a thin film transistor causes decrease in on current, a
technique for reducing parasitic resistance is considered. A thin film
transistor including an oxide semiconductor layer particularly has a
problem in that a high-resistance oxide film is formed in a connection
interface between the oxide semiconductor and metal.

[0018] A technique for suppressing off current of a thin film transistor
is also considered. A thin film transistor including an oxide
semiconductor layer particularly has a problem in that carriers which are
left in the oxide semiconductor layer affect transistor characteristics.
In addition, there is a problem in that impurities enter the inside of
the thin film transistor from the outside after long-term use, resulting
in change in transistor characteristics such as a threshold value.

[0019] In order to prevent increase in wiring resistance, the technique of
forming a low-resistance wiring layer with the use of copper (Cu) is
considered. However, since Cu easily diffuses into a semiconductor or
silicon oxide, the operation of a semiconductor device might be unstable
and yield might be significantly reduced.

[0020] An object of one embodiment of the present invention is to provide
a semiconductor device typified by a display device having higher display
quality, in which an adverse effect such as voltage drop, a defect in
signal writing to a pixel, a defect in grayscale, and the like due to
wiring resistance are prevented.

[0021] Another object of one embodiment of the present invention is to
realize high speed operation of a semiconductor device.

[0022] Another object of one embodiment of the present invention is to
realize reduction in power consumption of a semiconductor device.

[0023] Another object of one embodiment of the present invention is to
provide a thin film transistor and operates stably and a semiconductor
device which includes the thin film transistor.

[0024] Another object of one embodiment of the present invention is to
realize a semiconductor device having excellent yield.

[0025] In order to achieve the above objects, a semiconductor device
according to the present invention may have a structure where a
low-resistance wiring is connected to a thin film transistor in which a
source electrode and a drain electrode including metal with a high oxygen
affinity are connected to an oxide semiconductor layer with a suppressed
impurity concentration. Further, the thin film transistor using an oxide
semiconductor may be surrounded by an insulating film to be sealed.

[0026] That is, an embodiment of the present invention disclosed in this
specification is a semiconductor device including: a first insulating
layer including silicon nitride over a substrate; a gate wiring which is
formed using a first conductive layer with low resistance and is provided
over the first insulating layer; a gate electrode layer formed using the
first conductive layer; a second insulating layer including silicon
nitride over the gate electrode layer; a third insulating layer including
silicon oxide over the second insulating layer; an island-shaped oxide
semiconductor layer over the third insulating layer; second conductive
layers which are provided over the island-shaped oxide semiconductor
layer and function as a source electrode and a drain electrode; a fourth
insulating layer including silicon oxide over the second conductive
layers and the oxide semiconductor layer; a fifth insulating layer
including silicon nitride over the fourth insulating layer; a third
conductive layer that is electrically in contact with one of the second
conductive layers which function as the source electrode and the drain
electrode through an opening provided in the fourth insulating layer and
the fifth insulating layer; a sixth insulating layer including silicon
nitride which covers the third conductive layer and the fifth insulating
layer; and a fourth conductive layer that is electrically in contact with
the other of the second conductive layers which function as the source
electrode and the drain electrode through an opening provided in the
fourth insulating layer, the fifth insulating layer, and the sixth
insulating layer, wherein the second conductive layers have oxygen
affinity.

[0027] Another embodiment of the present invention disclosed in this
specification is the above semiconductor device in which the gate wiring
over the first insulating layer is formed using a conductive layer
including Cu and a conductive layer including metal with a high melting
point which covers the conductive layer including Cu, and in which the
gate electrode layer is formed using the metal with a high melting point.

[0028] Another embodiment of the present invention disclosed in this
specification is the above semiconductor device further including a
storage capacitor portion where the second insulating layer and the third
insulating layer are sandwiched between the first conductive layer and
the second conductive layer.

[0029] Another embodiment of the present invention disclosed in this
specification is the above semiconductor device in which the oxide
semiconductor layer includes at least one of indium, gallium, and zinc.

[0030] Another embodiment of the present invention disclosed in this
specification is the above semiconductor device in which the second
conductive layers include at least one element selected from W, Ta, Mo,
Ti, Cr, Al, Zr, and Ca.

[0031] Another embodiment of the present invention disclosed in this
specification is a method for manufacturing a semiconductor device,
including the steps of: forming a first insulating layer including
silicon nitride over a substrate; forming a gate wiring and a gate
electrode layer over the first insulating layer with the use of a first
conductive layer with low resistance; forming a second insulating layer
including silicon nitride over the gate electrode layer; forming a third
insulating layer including silicon oxide over the second insulating
layer; forming an oxide semiconductor layer over the third insulating
layer while the substrate is heated at higher than or equal to
100° C. and lower than or equal to 600° C.; forming second
conductive layers which function as a source electrode and a drain
electrode over the oxide semiconductor layer with the use of a conductive
layer with oxygen affinity; forming a fourth insulating layer including
silicon oxide over the second conductive layers and the oxide
semiconductor layer; forming a fifth insulating layer including silicon
nitride over the fourth insulating layer; forming an opening in the
fourth insulating layer and the fifth insulating layer; forming a third
conductive layer that is electrically in contact with one of the second
conductive layers which function as the source electrode and the drain
electrode through the opening; forming a sixth insulating layer including
silicon nitride which covers the third conductive layer and the fifth
insulating layer; forming an opening in the fourth insulating layer, the
fifth insulating layer, and the sixth insulating layer; and forming a
fourth conductive layer that is electrically in contact with the other of
the second conductive layers which function as the source electrode and
the drain electrode through the opening.

[0032] Note that a gate in this specification refers to the entire gate
electrode and gate wiring or part thereof. The gate wiring is a wiring
for electrically connecting a gate electrode of at least one transistor
to another electrode or another wiring, and includes a scan line in a
display device in its category, for example.

[0033] A source refers to the entire source region, source electrode, and
source wiring or part thereof. The source region indicates a region in a
semiconductor layer, where the resistivity is less than or equal to a
given value. The source electrode indicates part of a conductive layer,
which is connected to the source region. The source wiring is a wiring
for electrically connecting a source electrode of at least one transistor
to another electrode or another wiring. For example, in the case where a
signal line in a display device is electrically connected to a source
electrode, the source wiring includes the signal line in its category.

[0034] A drain refers to the entire drain region, drain electrode, and
drain wiring or part thereof. The drain region indicates a region in a
semiconductor layer, where the resistivity is less than or equal to a
given value. The drain electrode indicates part of a conductive layer,
which is connected to the drain region. The drain wiring is a wiring for
electrically connecting a drain electrode of at least one transistor to
another electrode or another wiring. For example, in the case where a
signal line in a display device is electrically connected to a drain
electrode, the drain wiring includes the signal line in its category.

[0035] In addition, in this document (the specification, the scope of
claims, the drawings, and the like), a source and a drain of a transistor
are interchanged depending on the structure, the operating conditions, or
the like of the transistor; therefore, it is difficult to determine which
is the source and which is the drain. Therefore, in this document (the
specification, the scope of claims, the drawings, and the like), one
terminal which is freely selected from the source and the drain is
referred to as one of the source and the drain, whereas the other
terminal is referred to as the other of the source and the drain.

[0036] Note that a light-emitting device in this specification means an
image display device, a light-emitting device, or a light source
(including a lighting device). In addition, the light-emitting device
includes any of the following modules in its category: a module in which
a connector such as a flexible printed circuit (FPC), a tape automated
bonding (TAB) tape, or a tape carrier package (TCP) is attached to a
light-emitting device; a module having a TAB tape or a TCP provided with
a printed wiring board at the end thereof; and a module having an
integrated circuit (IC) directly mounted over a substrate over which a
light-emitting element is formed by a chip on glass (COG) method.

[0037] A semiconductor device capable of high speed operation is provided.
In addition, a semiconductor device with low power consumption is
provided. Moreover, a highly reliable semiconductor device that operates
stably is provided.

BRIEF DESCRIPTION OF DRAWINGS

[0038] In the accompanying drawings:

[0039] FIGS. 1A and 1B illustrate a structure of a display device
according to an embodiment;

[0040] FIGS. 2A to 2C illustrate an example of a pixel structure of a
display device according to an embodiment;

[0041]FIG. 3 illustrates an example of a pixel structure of a display
device according to an embodiment;

[0042] FIGS. 4A to 4D illustrate a manufacturing process of a pixel
portion of a display device according to an embodiment;

[0043] FIGS. 5A to 5C illustrate a manufacturing process of a pixel
portion of a display device according to an embodiment;

[0044] FIGS. 6A to 6D illustrate a gray-tone mask and a half-tone mask;

[0045] FIGS. 7A to 7E illustrate a manufacturing process of a pixel
portion of a display device according to an embodiment;

[0046] FIGS. 8A to 8C illustrate an inverter circuit according to an
embodiment;

[0047] FIGS. 9A and 9B illustrate a protection circuit according to an
embodiment;

[0048]FIG. 10 illustrates a protection circuit according to an
embodiment;

[0049] FIGS. 11A and 11B illustrate a connection portion according to an
embodiment;

[0080] FIGS. 42A and 42B show structural models of metal atoms and oxygen
atoms in the vicinity of an interface between a tungsten film and an
oxide semiconductor film;

[0081] FIGS. 43A and 43B show structural models of metal atoms and oxygen
atoms in the vicinity of an interface between a molybdenum film and an
oxide semiconductor film;

[0082] FIGS. 44A and 44B show structural models of metal atoms and oxygen
atoms in the vicinity of an interface between a titanium film and an
oxide semiconductor film;

[0083]FIG. 45 shows a crystal structure of titanium dioxide having a
rutile structure;

[0084]FIG. 46 shows a density of states of titanium dioxide having a
rutile structure;

[0085]FIG. 47 shows a density of states of titanium dioxide in an
oxygen-deficient state;

[0086] FIG. 48 shows a density of states of titanium monoxide; and

[0087]FIG. 49 is a band diagram illustrating one embodiment of the
present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0088] Embodiments will be described in detail with reference to the
drawings. Note that the present invention is not limited to the following
description, and it is easily understood by those skilled in the art that
the mode and detail can be changed in various ways without departing from
the spirit and scope of the present invention. Therefore, the present
invention should not be construed as being limited to the description in
the embodiments below. Note that in the structures of the invention
described below, the same portions or portions having similar functions
are denoted by the same reference numerals in different drawings, and
description of such portions is not repeated.

Embodiment 1

[0089] In this embodiment, one embodiment of a display device in which a
pixel portion and a semiconductor element that includes an oxide
semiconductor and is provided in the periphery of the pixel portion are
formed will be described with reference to FIGS. 1A and 1B.

[0090]FIG. 1A illustrates a structure of a display device 30. The display
device 30 includes a gate terminal portion 7 and a source terminal
portion 8 over a substrate 100. The display device 30 is provided with
gate wirings (20_1 to 20--n (note that n is a natural number))
including the gate wiring 20_1 and the gate wiring 20_2, and source
wirings (60_1 to 60--m (note that m is a natural number)) including
the source wiring 60_1 and the source wiring 60_2. Further, in a pixel
region 94 of the display device 30, pixels 93 are arranged in matrix.
Note that each of the pixels 93 is connected to at least one gate wiring
and one source wiring.

[0091] Further, the display device 30 includes a common wiring 44, a
common wiring 45, a common wiring 46, and a common wiring 65. For
example, the common wiring 45 is connected to the common wiring 65
through a connection portion 95. The common wirings are electrically
connected to each other to have the same potential.

[0092] In addition, the common wiring 44, the common wiring 45, the common
wiring 46, and the common wiring 65 are connected to a terminal 71, a
terminal 75, a terminal 81, and a terminal 85, respectively. The common
wirings each include a common connection portion 96 which can be
electrically connected to a counter substrate.

[0093] Further, each of gate signal line terminals (70_1 to 70--i
(note that i is a natural number)) of the gate terminal portion 7 is
connected to a gate driver circuit 91 (hereinafter also referred to as a
scan line driver circuit) and connected to the common wiring 46 through a
protection circuit 97. In addition, a terminal 74 is connected to the
gate driver circuit 91 and connects an external power source (not
illustrated) to the gate driver circuit 91. Note that each of the gate
wirings (20_1 to 20--n (note that n is a natural number)) is
connected to the common wiring 65 through the protection circuit 97.

[0094] Further, each of source signal line terminals (80_1 to 80--k
(note that k is a natural number)) of the source terminal portion 8 is
connected to a source driver circuit 92 (hereinafter also referred to as
a signal line driver circuit), and connected to the common wiring 44
through the protection circuit 97. In addition, a terminal 84 is
connected to the source driver circuit 92 and connects an external power
source (not illustrated) to the source driver circuit 92. Each of the
source wirings (60_1 to 60--m (note that m is a natural number)) is
connected to the common wiring 45 through the protection circuit 97.

[0095] The gate driver circuit and the source driver circuit can be formed
at the same time as the pixel region with the use of a thin film
transistor disclosed in this specification. Moreover, one or both of the
gate driver circuit and the source driver circuit may be formed over a
substrate which is separately prepared with the use of a single crystal
semiconductor film or a polycrystalline semiconductor film, and then
mounted by a COG method, a wire bonding method, a TAB method, or the
like.

[0096] An example of an equivalent circuit that can be applied to the
pixel 93 is illustrated in FIG. 1B. The equivalent circuit illustrated in
FIG. 1B is an example in the case where a liquid crystal element is used
as a display element in the pixel 93.

[0097] Next, an example of a pixel structure of the display device
illustrated in FIGS. 1A and 1B is described with reference to FIGS. 2A to
2C. FIG. 2A is a top view illustrating a plan structure of the pixel, and
FIGS. 2B and 2C are cross-sectional views each illustrating a
stacked-layer structure of the pixel. Note that chain lines A1-A2, B1-B2,
and C1-C2 in FIG. 2A correspond to cross sections A1-A2, B1-B2, and C1-C2
in FIG. 2B, respectively. Chain line D1-D2 in FIG. 2A corresponds to
cross section D1-D2 in FIG. 2c.

[0098] In cross section A1-A2 and cross section D1-D2, stacked-layer
structures of a thin film transistor 250 used in the pixel portion are
illustrated. The thin film transistor 250 is one embodiment of a thin
film transistor having a bottom gate structure.

[0099] In cross section A1-A2 and cross section D1-D2, an insulating layer
201 provided over a substrate 200, a gate wiring 202 provided over the
insulating layer 201, a gate wiring 203 provided over the gate wiring
202, an insulating layer 204 provided over the gate wiring 203, a
semiconductor layer 205 provided over the insulating layer 204, a pair of
electrodes 207a and 207b provided over the semiconductor layer 205, an
insulating layer 208 provided over the electrode 207a, the electrode
207b, and the semiconductor layer 205, a source wiring 209 which is in
contact with the electrode 207a through an opening provided in the
insulating layer 208, a source wiring 210 provided over the source wiring
209, an insulating layer 211 provided over the source wiring 210, and an
electrode 212 which is in contact with the electrode 207b through an
opening provided in the insulating layer 211 and the insulating layer 208
are illustrated.

[0100] Further, in cross section B1-B2, a stacked-layer structure of a
storage capacitor (also referred to as a Cs capacitor) is illustrated. In
cross section B1-B2, the insulating layer 201 over the substrate 200, a
storage capacitor wiring 213 over the insulating layer 201, a storage
capacitor wiring 214 over the storage capacitor wiring 213, the
insulating layer 204 over the storage capacitor wiring 214, the electrode
207b over the insulating layer 204, the insulating layer 208 over the
electrode 207b, the insulating layer 211 over the insulating layer 208,
and the electrode 212 over the insulating layer 211 are illustrated.

[0101] Further, in cross section C1-C2, a stacked-layer structure in a
wiring intersection of the gate wiring and the source wiring is
illustrated. In cross section C1-C2, the insulating layer 201 over the
substrate 200, the gate wiring 202 over the insulating layer 201, the
gate wiring 203 over the gate wiring 202, the insulating layer 204 over
the gate wiring 203, the insulating layer 208 over the insulating layer
204, the source wiring 209 over the insulating layer 208, the source
wiring 210 over the source wiring 209, and the insulating layer 211 over
the source wiring 210 are illustrated. Note that in the wiring
intersection, a semiconductor layer may be formed between the insulating
layer 204 and the insulating layer 208.

[0102] In addition, one embodiment of the present invention is not limited
to the pixel structure illustrated in FIG. 2B. FIG. 3 illustrates an
example of a pixel structure different from that of FIG. 2B. A thin film
transistor 251 illustrated in FIG. 3 is one embodiment of a thin film
transistor having a bottom gate structure and can be called a channel
protective thin film transistor.

[0103] The thin film transistor 251 includes the insulating layer 201
provided over the substrate 200, the gate wiring 202 provided over the
insulating layer 201, the gate wiring 203 provided over the gate wiring
202, the insulating layer 204 provided over the gate wiring 203, the
semiconductor layer 205 provided over the insulating layer 204, a channel
protective layer 225 provided over the semiconductor layer 205, the pair
of electrodes 207a and 207b provided over the channel protective layer
225, the insulating layer 208 provided over the electrode 207a, the
electrode 207b, and the semiconductor layer 205, the source wiring 209
which is in contact with the electrode 207a through an opening provided
in the insulating layer 208, the source wiring 210 provided over the
source wiring 209, the insulating layer 211 provided over the source
wiring 210, and the electrode 212 which is in contact with the electrode
207b through an opening provided in the insulating layer 211 and the
insulating layer 208.

[0104] FIGS. 39A to 39C illustrate a pixel structure different from that
in FIG. 2B or FIG. 3. A thin film transistor 252 illustrated as an
example in FIGS. 39A to 39C includes the insulating layer 201 provided
over the substrate 200, the gate wiring 203 provided over the insulating
layer 201, the insulating layer 204 provided over the gate wiring 203,
the semiconductor layer 205 provided over the insulating layer 204, the
pair of electrodes 207a and 207b provided over the semiconductor layer
205, the insulating layer 208 provided over the electrode 207a, the
electrode 207b, and the semiconductor layer 205, the source wiring 209
which is in contact with the electrode 207a through an opening provided
in the insulating layer 208, the insulating layer 211 provided over the
source wiring 209, and the electrode 212 which is in contact with the
electrode 207b through an opening provided in the insulating layer 211
and the insulating layer 208.

[0105] Note that a wiring material may be selected as appropriate in
accordance with the performance needed for the display device. For
example, only the source wiring 209 which needs to have a higher
conduction property than the gate wiring may be formed using the wiring
including Cu.

[0106] In the case where a conductive film mainly including Al as a wiring
material with low electric resistance is used for the gate wiring 203, a
gate electrode of the thin film transistor can have the same structure as
the gate wiring 203 as illustrated in FIGS. 39A to 39C.

[0107] A storage capacitor of the pixel which is described as an example
in this embodiment is formed so that the insulating layer 204 is
sandwiched between the electrode 207b and the storage capacitor wiring
which is formed using the same layers as the gate wirings. Since the
electrode 207b is close to the storage capacitor wiring in the thickness
direction as compared to the electrode 212 and the source wiring 210, the
electrode 207b is suitable for the storage capacitor.

[0108] By forming the gate wiring 202 and the source wiring 210 with the
use of a conductive material including Cu, increase in wiring resistance
can be prevented. Further, when the gate wiring 203 is formed using a
conductive material including an element with a higher melting point than
Cu, such as W, Ta, Mo, Ti, or Cr, so as to be in contact with and cover
the gate wiring 202, migration of the gate wiring 202 is suppressed and
reliability of the semiconductor device can be improved. Furthermore, by
providing insulating layers including silicon nitride as the insulating
layers over and under a conductive layer including Cu so that the gate
wiring 202 including Cu may be sandwiched between or surrounded by the
insulating layers, Cu diffusion can be prevented.

[0109] In addition, the gate wiring 202 is provided in such a manner that
it does not overlap with the semiconductor layer 205 in which a channel
of the thin film transistor is formed, and part of the gate wiring 203
which is in contact with the gate wiring 202 is extended to overlap with
the semiconductor layer 205 and function as a gate electrode. With such a
structure, an influence of Cu included in the gate wiring 202 on the thin
film transistor can be prevented.

[0110] At least the insulating layer 204 and the insulating layer 208 are
sandwiched between the gate wiring and the source wiring in the wiring
intersection, whereby the distance in the thickness direction between the
wirings can be increased. As a result, parasitic capacitance in the
wiring intersection can be reduced.

[0111] Note that this embodiment can be combined with any of the other
embodiments in this specification as appropriate.

Embodiment 2

[0112] In this embodiment, a manufacturing process of a pixel portion in
the display device described in Embodiment 1 will be described with
reference to FIGS. 4A to 4D and FIGS. 5A to 5C. Note that cross section
A1-A2, cross section B1-B2, cross section C1-C2, and cross section D1-D2
in FIGS. 4A to 4D and FIGS. 5A to 5C are cross-sectional views taken
along chain lines A1-A2, B1-B2, C1-C2, and D1-D2 in FIG. 2A,
respectively.

[0113] First, the insulating layer 201 including silicon nitride is formed
to a thickness of greater than or equal to 50 nm and less than or equal
to 300 nm, preferably greater than or equal to 100 nm and less than or
equal to 200 nm over the substrate 200. As the substrate 200, in addition
to a glass substrate and a ceramic substrate, a plastic substrate or the
like with heat resistance to withstand a process temperature in this
manufacturing process can be used. In the case where the substrate does
not need a light-transmitting property, a metal substrate, such as a
stainless steel alloy substrate, provided with an insulating film on its
surface may be used. As a glass substrate, for example, an alkali-free
glass substrate of barium borosilicate glass, aluminoborosilicate glass,
aluminosilicate glass, or the like may be used. Alternatively, a quartz
substrate, a sapphire substrate, or the like can be used. Further, as the
substrate 200, a glass substrate with any of the following sizes can be
used: the 3rd generation (550 mm×650 mm), the 3.5th generation (600
mm×720 mm or 620 mm×750 mm), the 4th generation
(680×880 mm or 730 mm×920 mm), the 5th generation (1100
mm×1300 mm), the 6th generation (1500 mm×1850 mm), the 7th
generation (1870 mm×2200 mm), the 8th generation (2200
mm×2400 mm), the 9th generation (2400 mm×2800 mm or 2450
mm×3050 mm), or the 10th generation (2950 mm×3400 mm). In
this embodiment, aluminoborosilicate glass is used for the substrate 200.

[0114] The insulating layer 201 can be formed as a single layer or a
stacked layer of a silicon nitride film and/or a silicon nitride oxide
film. Note that in this specification, silicon nitride oxide refers to
silicon that includes more nitrogen than oxygen and, in the case where
measurements are performed using RBS and HFS, includes oxygen, nitrogen,
silicon, and hydrogen at concentrations ranging from 5 at. % to 30 at. %,
20 at. % to 55 at. %, 25 at. % to 35 at. %, and 10 at. % to 30 at. %,
respectively. The insulating layer 201 can be formed by a sputtering
method, a CVD method, a coating method, a printing method, or the like as
appropriate. In this embodiment, a 100-nm-thick silicon nitride film is
formed as the insulating layer 201. Note that the film may be doped with
phosphorus (P) or boron (B).

[0115] Then, a conductive film including Cu is formed to a thickness of
greater than or equal to 100 nm and less than or equal to 500 nm,
preferably greater than or equal to 200 nm and less than or equal to 300
nm, over the insulating layer 201 by a sputtering method, a vacuum
evaporation method, or a plating method. A mask is formed over the
conductive film by a photolithography method, an inkjet method, or the
like and the conductive film is etched using the mask; thus, the gate
wiring 202 and the storage capacitor wiring 213 can be formed. In order
to improve adhesion of the gate wiring 202, a metal layer including W,
Ta, Mo, Ti, Cr, or the like, an alloy layer including any of these in
combination, or a layer of a nitride or an oxide of any of these may be
formed between the insulating layer 201 and the gate wiring 202.

[0116] Note that a photomask is not used when the resist mask is formed by
an inkjet method, which results in reducing manufacturing cost. Further,
when a conductive nanopaste of copper or the like is discharged over the
substrate by an inkjet method and baked, the gate wiring 202 and the
storage capacitor wiring 213 can be formed at low cost.

[0117] In this embodiment, a 250-nm-thick Cu film is formed over the
insulating layer 201 and the Cu film is selectively etched using a resist
mask formed by a first photolithography step, whereby the gate wiring 202
and the storage capacitor wiring 213 are formed (see FIG. 4A).

[0118] Then, a conductive film of an element such as W, Ta, Mo, Ti, or Cr,
which has a higher melting point than Cu, or an alloy or the like
including a combination of any of these elements is formed to a thickness
of greater than or equal to 5 nm and less than or equal to 200 nm,
preferably greater than or equal to 10 nm and less than or equal to 100
nm, by a sputtering method, a vacuum evaporation method, or the like over
the gate wiring 202. The conductive film is not limited to a single-layer
film including any of the above elements and can be a stacked-layer film
of two or more layers. In this embodiment, a 200-nm-thick single layer of
tungsten is formed as the conductive film.

[0119] Then, a mask is formed over the conductive film by a
photolithography method, an inkjet method, or the like, and then the
conductive film is etched using the mask; thus, the gate wiring 203 and
the storage capacitor wiring 214 can be formed. In this embodiment, the
conductive film is selectively etched using a resist mask formed by a
second photolithography step, whereby the gate wiring 203 and the storage
capacitor wiring 214 are formed (see FIG. 4B).

[0120] A gate wiring and a storage capacitor wiring are formed to have a
structure in which a conductive layer including an element having a
higher melting point than Cu covers a conductive material including Cu.
With such a structure, migration of the layer including Cu is suppressed
and thus reliability of the semiconductor device can be improved.

[0121] Then, the insulating layer 204 functioning as a gate insulating
layer is formed to a thickness of greater than or equal to 50 nm and less
than or equal to 800 nm, preferably greater than or equal to 100 nm and
less than or equal to 600 nm over the gate wiring 203. In this
embodiment, the insulating layer 204 is formed by stacking an insulating
layer 204a and an insulating layer 204b in this order. A silicon nitride
(SiNy (y>0)) layer is formed as the insulating layer 204a by a
sputtering method, and a silicon oxide (SiOx (x>0)) layer is
formed over the insulating layer 204a as the insulating layer 204b; thus,
the gate insulating layer 204 with a thickness of 100 nm is formed.

[0122] The insulating layer 204 also functions as a protective layer. By
providing insulating layers including silicon nitride as the insulating
layers over and under the conductive layer including Cu so that the
conductive layer including Cu may be sandwiched between or surrounded by
the insulating layers, Cu diffusion can be prevented.

[0123] Next, the semiconductor layer 205 is formed over the insulating
layer 204. As an oxide semiconductor film serving as the semiconductor
layer 205, an In--Ga--Zn--O-based oxide semiconductor film, an
In--Sn--Zn--O-based oxide semiconductor film, an In--Al--Zn--O-based
oxide semiconductor film, a Sn--Ga--Zn--O-based oxide semiconductor film,
an Al--Ga--Zn--O-based oxide semiconductor film, a Sn--Al--Zn--O-based
oxide semiconductor film, an In--Zn--O-based oxide semiconductor film, a
Sn--Zn--O-based oxide semiconductor film, an Al--Zn--O-based oxide
semiconductor film, an In--O-based oxide semiconductor film, a
Sn--O-based oxide semiconductor film, or a Zn--O-based oxide
semiconductor film is used. In addition, the oxide semiconductor film can
be formed by a sputtering method in a rare gas (typically argon)
atmosphere, an oxygen atmosphere, or a mixed atmosphere of a rare gas
(typically argon) and oxygen.

[0124] In the case of using a sputtering method, deposition is performed
using a target including silicon oxide (SiO2) at greater than or
equal to 2 weight % and less than or equal to 10 weight % so that
SiOx (x>0) which inhibits crystallization may be included in the
oxide semiconductor film, whereby crystallization can be suppressed. This
is particularly effective in the case of performing heat treatment in a
subsequent step.

[0125] Here, deposition is performed using a metal oxide target including
In, Ga, and Zn (composition ratio:
In2O3:Ga2O3:ZnO=1:1:1 [mol %], In:Ga:Zn=1:1:0.5 [at.
%]) under conditions where the distance between the substrate and the
target is 100 mm, the pressure is 0.6 Pa, the direct-current (DC) power
source is 0.5 kW, and the atmosphere is an oxygen atmosphere (of an
oxygen flow rate of 100%). Note that a pulsed direct-current (DC) power
source is preferably used because powder substances (also referred to as
particles or dust) generated during deposition can be reduced and the
film thickness can be uniform. In this embodiment, as the oxide
semiconductor film, an In--Ga--Zn--O-based film is formed by a sputtering
method with the use of an In--Ga--Zn--O-based metal oxide target.

[0126] The filling rate of the oxide semiconductor target is higher than
or equal to 90% and lower than or equal to 100%, preferably higher than
or equal to 95% and lower than or equal to 99.9%. With the use of the
oxide semiconductor target with a high filling rate, a dense oxide
semiconductor film is formed.

[0127] The oxide semiconductor film preferably has a thickness of greater
than or equal to 5 nm and less than or equal to 30 nm. Note that an
appropriate thickness differs depending on an oxide semiconductor
material, and the thickness may be set as appropriate depending on the
material.

[0128] In this embodiment, the oxide semiconductor film is successively
formed over the gate insulating layer 204. A multi-chamber sputtering
apparatus used here is provided with a target of silicon or silicon oxide
(artificial quarts), and a target for the oxide semiconductor film. A
deposition chamber provided with the target for the oxide semiconductor
film is further provided with at least a cryopump as an evacuation unit.
Note that a turbo molecular pump may be used instead of the cryopump, and
a cold trap may be provided so that moisture or the like may be adsorbed
onto an inlet of the turbo molecular pump.

[0129] In the deposition chamber which is evacuated with the cryopump, a
hydrogen atom, a compound including a hydrogen atom such as H2O, a
compound including a carbon atom, and the like are removed, whereby the
concentration of an impurity included in the oxide semiconductor film
formed in the deposition chamber can be reduced.

[0130] The oxide semiconductor film is formed in a state where the
substrate is heated. In this embodiment, the substrate temperature is set
at higher than or equal to 100° C. and lower than or equal to
600° C., preferably higher than or equal to 200° C. and
lower than or equal to 400° C. By forming the oxide semiconductor
film in the state where the substrate is heated, the concentration of an
impurity included in the formed oxide semiconductor film can be reduced.
Further, in order to control the composition ratio of elements included
in the oxide semiconductor, it is preferable that the heating temperature
be as low as possible. In the case of an oxide semiconductor including
zinc, for example, the rate of zinc included in a semiconductor layer
formed at a high temperature is lowered because zinc has high vapor
pressure. Note that the sputtering conditions are set to be as mild as
possible so as not to damage the oxide semiconductor film.

[0131] Examples of a sputtering method include an RF sputtering method in
which a high-frequency power source is used as a sputtering power source,
a DC sputtering method, and a pulsed DC sputtering method in which a bias
is applied in a pulsed manner. An RF sputtering method is mainly used in
the case of forming an insulating film, and a DC sputtering method is
mainly used in the case of forming a metal conductive film.

[0132] In addition, there is also a multi-source sputtering apparatus in
which a plurality of targets of different materials can be set. With the
multi-source sputtering apparatus, films of different materials can be
formed to be stacked in the same chamber, or a film of plural kinds of
materials can be formed by electric discharge at the same time in the
same chamber.

[0133] In addition, there are a sputtering apparatus provided with a
magnet system inside the chamber and used for a magnetron sputtering
method, and a sputtering apparatus used for an ECR sputtering method in
which plasma generated with the use of microwaves is used without using
glow discharge.

[0134] Furthermore, as a deposition method by sputtering, there are also a
reactive sputtering method in which a target substance and a sputtering
gas component are chemically reacted with each other during deposition to
form a thin compound film thereof, and a bias sputtering method in which
voltage is also applied to a substrate during deposition.

[0135] Note that before the oxide semiconductor film is formed by a
sputtering method, reverse sputtering in which an argon gas is introduced
and plasma is generated is preferably performed to remove dust attaching
to a surface of the gate insulating layer 204. The reverse sputtering
refers to a method in which an RF power source is used for application of
voltage to a substrate side in an argon atmosphere and plasma is
generated in the vicinity of the substrate to modify a surface. Note that
instead of an argon atmosphere, a nitrogen atmosphere, a helium
atmosphere, an oxygen atmosphere, or the like may be used.

[0136] Then, a mask is formed over the oxide semiconductor film by a
photolithography method, an inkjet method, or the like and the oxide
semiconductor film is selectively etched using the mask to provide the
semiconductor layer 205 having an island shape. In this embodiment, the
oxide semiconductor film is selectively etched using a resist mask formed
by a third photolithography step to be the semiconductor layer 205 having
an island shape (see FIG. 4c).

[0137] Then, although not illustrated in FIGS. 2A to 2C, FIG. 3, FIGS. 4A
to 4D, and FIGS. 5A to 5C, an opening (also referred to as a contact
hole) for connecting the gate wiring 203 to the electrode 207a or the
electrode 207b which will be described later is formed in the insulating
layer 204. The contact hole is formed by forming a mask over the
insulating layer 204 by a photolithography method, an inkjet method, or
the like, and then selectively etching the insulating layer 204 using the
mask. Here, the insulating layer 204 is selectively etched using a resist
mask formed by a fourth photolithography step, whereby a contact hole is
formed.

[0138] Note that the contact hole may be formed by the fourth
photolithography step after the formation of the insulating layer 204 and
before the formation of the semiconductor layer 205.

[0139] Next, a conductive film is formed over the oxide semiconductor
layer 205. For the conductive film, W, Ta, Mo, Ti, Cr, Al, an alloy
including any of these elements in combination, or the like can be used.
Alternatively, a metal nitride such as titanium nitride, tantalum
nitride, or tungsten nitride may be used for the conductive film. Note
that the conductive film can have a stacked-layer structure of two or
more layers.

[0140] Note that in the case of performing heat treatment at 200°
C. to 600° C., the conductive film preferably has heat resistance
to withstand this heat treatment. For example, it is preferable to use an
aluminum alloy to which an element which prevents hillocks is added, or a
conductive film stacked with a heat-resistant conductive film.

[0141] For the conductive film which is in contact with the oxide
semiconductor layer 205, a material including metal with high oxygen
affinity is preferable.

[0142] As the metal with high oxygen affinity, one or more materials
selected from titanium (Ti), manganese (Mn), aluminum (Al), magnesium
(Mg), zirconium (Zr), beryllium (Be), and thorium (Th) are preferable. In
this embodiment, a titanium film is used.

[0143] When the oxide semiconductor layer and the conductive film with
high oxygen affinity are formed in contact with each other, the carrier
density in the vicinity of the interface is increased and a
low-resistance region is formed, whereby the contact resistance between
the oxide semiconductor layer and the conductive film can be reduced.
This is because the conductive film with high oxygen affinity extracts
oxygen from the oxide semiconductor layer and thus either or both of a
layer which includes metal in the oxide semiconductor layer in excess
(such a layer is referred to as a composite layer) and an oxidized
conductive film are formed in the interface between the oxide
semiconductor layer and the conductive film. For example, in a structure
where an In--Ga--Zn--O-based oxide semiconductor layer is in contact with
a titanium film, an indium-excess layer and a titanium oxide layer are
formed in the vicinity of the interface where the oxide semiconductor
layer is in contact with the titanium film in some cases. In other cases,
one of the indium-excess layer and the titanium oxide layer is formed in
the vicinity of the interface where the oxide semiconductor layer is in
contact with the titanium film. The indium-excess layer which is an
oxygen-deficient In--Ga--Zn--O-based oxide semiconductor layer has high
electric conductivity; therefore, the contact resistance between the
oxide semiconductor layer and the conductive film can be reduced.

[0144] Note that a titanium oxide film having conductivity may be used as
the conductive film which is in contact with the oxide semiconductor
layer. In that case, in the structure where the In--Ga--Zn--O-based oxide
semiconductor layer is in contact with the titanium oxide film, an
indium-excess layer might be formed in the vicinity of the interface
where the oxide semiconductor layer is in contact with the titanium oxide
film.

[0145] Note that a phenomenon in which, in a channel-etched thin film
transistor using the above In--Ga--Zn--O-based oxide semiconductor film
as an active layer of the thin film transistor, a layer which includes
indium at a higher concentration than the other region (an In-rich layer)
and a titanium oxide (TiOx) film are formed in the vicinity of the
interface between the In--Ga--Zn--O-based oxide semiconductor film and a
metal film used for a source electrode and a drain electrode will be
described in detail in Embodiment 14.

[0146] The conductive film is formed to a thickness of greater than or
equal to 100 nm and less than or equal to 500 nm, preferably greater than
or equal to 200 nm and less than or equal to 300 nm. As a formation
method of the conductive film, a sputtering method, a vacuum evaporation
method (such as an electron beam evaporation method), an arc discharge
ion plating method, or a spray method is employed. Alternatively, the
conductive film may be formed by discharging a conductive nanopaste of
silver, gold, copper, or the like by a screen printing method, an inkjet
method, or the like and baking the nanopaste.

[0147] Then, a mask is formed over the conductive film by a
photolithography method, an inkjet method, or the like and the conductive
film is etched using the mask; thus, the electrode 207a serving as a
source electrode and the electrode 207b serving as the drain electrode
can be formed. In this embodiment, a 200-nm-thick Ti film is formed by a
sputtering method as the conductive film, and then the conductive film is
selectively etched by a dry etching method using a resist mask formed by
a fifth photolithography step, whereby the electrodes 207a and 207b are
formed.

[0148] By the fifth photolithography step, only a portion of the
conductive film which is over and in contact with the oxide semiconductor
layer is removed. When an ammonia peroxide mixture (hydrogen
peroxide:ammonia:water=5:2:2 in a weight ratio), or the like is used as
an alkaline etchant so that only the portion of the conductive film which
is over and in contact with the oxide semiconductor layer is removed, it
is possible to remove the metal conductive film selectively and to leave
the oxide semiconductor layer including an In--Ga--Zn--O-based oxide
semiconductor.

[0149] Further, an exposed region of the oxide semiconductor layer is
etched by the fifth photolithography step in some cases, depending on the
etching condition. In that case, the thickness of the oxide semiconductor
layer in a region between the source electrode layer and the drain
electrode layer (a region between reference numerals 207a and 207b) is
smaller than the thickness of the oxide semiconductor layer in a region
overlapping with the source electrode layer over the gate wiring 203 or
the thickness of the oxide semiconductor layer in a region overlapping
with the drain electrode layer over the gate wiring 203 (see FIG. 4D).

[0150] Next, the insulating layer 208 is formed over the gate insulating
layer 204 and the oxide semiconductor layer 205. The insulating layer 208
does not include impurities such as moisture, hydrogen ions, or OHand is formed using an inorganic insulating film which prevents entry of
these from the outside. In addition, the insulating layer 208 is formed
using an inorganic insulating film which suppresses migration of a layer
including Cu from a source wiring formed in a subsequent step. In this
embodiment, the insulating layer 208 is formed by stacking an insulating
layer 208a and an insulating layer 208b in this order.

[0151] An oxide insulating film is used for the insulating layer 208a
which is in contact with the oxide semiconductor layer 205. The
insulating layer 208a can be formed to a thickness of at least 1 nm by a
method with which impurities such as water or hydrogen are not mixed into
the oxide insulating film, such as a sputtering method, as appropriate.
Typically, a single layer or a stacked layer of any of a silicon oxide
film, a silicon nitride oxide film, an aluminum oxide film, an aluminum
oxynitride film, and the like is used for the formation.

[0152] The substrate temperature in the deposition may be higher than or
equal to room temperature and lower than or equal to 300° C., and
is 100° C. in this embodiment. The silicon oxide film can be
formed by a sputtering method in a rare gas (typically argon) atmosphere,
an oxygen atmosphere, or an atmosphere of a rare gas (typically argon)
and oxygen. Note that an oxide insulating film formed by a sputtering
method is distinctively dense and even a single layer of the oxide
insulating film can be used as a protective film for suppressing a
phenomenon in which impurities are diffused into a layer in contact
therewith. In addition, a target doped with phosphorus (P) or boron (B)
can be used so that phosphorus (P) or boron (B) is added to the oxide
insulating film.

[0153] As a target, a silicon oxide target or a silicon target can be
used, and a silicon target is particularly preferable. A silicon oxide
film formed by a sputtering method in an atmosphere of oxygen and a rare
gas with the use of a silicon target includes a large number of dangling
bonds of silicon atoms or oxygen atoms.

[0154] Since the insulating layer 208a includes many dangling bonds,
impurities included in the oxide semiconductor layer 205 are more likely
to diffuse into the insulating layer 208a through the interface where the
oxide semiconductor layer 205 is in contact with the insulating layer
208a. Specifically, a hydrogen atom, a compound including a hydrogen atom
such as H2O, a compound including a carbon atom, or the like in the
oxide semiconductor layer 205 is more likely to diffuse and move into the
insulating layer 208a and fixed in the insulating layer 208a.

[0155] In this embodiment, deposition is performed by a pulsed DC
sputtering method using a columnar polycrystalline silicon target (the
resistivity is 0.01 Ωcm) which is doped with boron and has a purity
of 6N, under conditions where the distance between substrate and target
(T-S distance) is 89 mm, the pressure is 0.4 Pa, the direct-current (DC)
power source is 6 kW, and the atmosphere is an oxygen atmosphere (of an
oxygen flow rate of 100%). The film thickness thereof is 300 nm.

[0156] At this stage, a region where the oxide semiconductor layer 205 is
in contact with the oxide insulating layer 208a is formed. A region of
the oxide semiconductor layer 205, which overlaps with the gate electrode
and sandwiched between and in contact with the gate insulating layer 204
and the insulating layer 208a, serves as a channel formation region. In
addition, the insulating layer 208a functions as a channel protective
layer.

[0157] An insulating film including nitrogen is used for the insulating
layer 208b which is formed over the insulating layer 208a. The insulating
layer 208b is formed to a thickness of at least 1 nm by a method with
which impurities such as water or hydrogen are not mixed into the
insulating film, such as a sputtering method, as appropriate. Typically,
a silicon nitride film, a silicon nitride oxide film, an aluminum nitride
film, or the like is used. In this embodiment, a silicon nitride film is
formed as the insulating layer 208b by an RF sputtering method.

[0158] In this embodiment, a 400-nm-thick silicon nitride film is formed
as the insulating layer 208b.

[0159] Then, the opening 216 (also referred to as a contact hole) for
connecting the electrode 207a to the source wiring 209 is formed in the
insulating layer 208. The contact hole is formed by forming a mask over
the insulating layer 208 by a photolithography method, an inkjet method,
or the like, and then selectively etching the insulating layer 208 using
the mask. In this embodiment, the insulating layer 208 is selectively
etched using a resist mask formed by a sixth photolithography step,
whereby a contact hole is formed.

[0160] Then, a conductive film for the formation of the source wiring 209
is formed using an element such as W, Ta, Mo, Ti, or Cr which has a
higher melting point than Cu or an alloy or the like including a
combination of any of these elements by a sputtering method, a vacuum
evaporation method, or the like with a thickness of greater than or equal
to 5 nm and less than or equal to 200 nm, preferably greater than or
equal to 10 nm and less than or equal to 100 nm. Alternatively, a film of
tantalum nitride (TaN), titanium nitride (TiN), or the like may be formed
by a reactive sputtering method.

[0161] Then, a conductive film including Cu is formed to a thickness of
greater than or equal to 100 nm and less than or equal to 500 nm,
preferably greater than or equal to 200 nm and less than or equal to 300
nm by a sputtering method, a vacuum evaporation method, or a plating
method. A mask is formed over the conductive film by a photolithography
method, an inkjet method, or the like and the conductive film including
Cu and the conductive film for the formation of the source wiring 209 are
etched using the mask; thus the source wiring 209 and the source wiring
210 can be formed.

[0162] In this embodiment, a 50-nm-thick titanium nitride film is used as
the conductive film for the formation of the source wiring 209 and a
250-nm-thick Cu film is used as the conductive film for the formation of
the source wiring 210, and the conductive films are selectively etched
using a resist mask formed by a seventh photolithography step, whereby
the source wiring 209 and the source wiring 210 are formed (see FIG. 5A).

[0163] By forming a source wiring with a stacked-layer structure having a
layer including Cu and a layer including an element which has a higher
melting point than Cu, migration of the layer including Cu is suppressed;
thus, the reliability of the semiconductor device can be improved.
Further, a structure is also acceptable in which another layer including
an element which has a higher melting point than Cu is formed over the
source wiring 210 so that the layer including Cu is sandwiched between
the layers including an element which has a higher melting point than Cu.
Note that the source wiring may be formed using only the layer including
Cu, depending on the usage environment or the usage condition of the
semiconductor device.

[0164] Then, the insulating layer 211 is formed to a thickness of greater
than or equal to 50 nm and less than or equal to 300 nm, preferably
greater than or equal to 100 nm and less than or equal to 200 nm. The
insulating layer 211 can be formed by a method similar to the method for
forming the insulating layer 201. In this embodiment, a 10-nm-thick
silicon nitride film is formed as the insulating layer 211. The
insulating layer 211 also serves as a protective layer. By providing
insulating layers including silicon nitride as the insulating layers over
and under the conductive layer including Cu so that the conductive layer
including Cu may be sandwiched between or surrounded by the insulating
layers, Cu diffusion can be prevented (see FIG. 5B).

[0165] Then, a contact hole for connecting the electrode 207b to the
electrode 212 serving as a pixel electrode is formed in the insulating
layer 211 and the insulating layer 208. The contact hole is formed by
forming a mask over the insulating layer 211 by a photolithography
method, an inkjet method, or the like, and then selectively etching the
insulating layers 211 and 208 using the mask. In this embodiment, the
insulating layers 211 and 208 are selectively etched using a resist mask
formed by an eighth photolithography step, whereby a contact hole (the
opening 217) is formed.

[0166] Then, a light-transmitting conductive film is formed to a thickness
of greater than or equal to 30 nm and less than or equal to 200 nm,
preferably greater than or equal to 50 nm and less than or equal to 100
nm by a sputtering method, a vacuum evaporation method, or the like. A
mask is formed over the conductive film by a photolithography method, an
inkjet method, or the like, and then the conductive film is etched using
the mask; thus, the electrode 212 which serves as a pixel electrode is
formed.

[0167] For the light-transmitting conductive film, a light-transmitting
conductive material such as indium oxide including tungsten oxide, indium
zinc oxide including tungsten oxide, indium oxide including titanium
oxide, indium tin oxide including titanium oxide, indium tin oxide
(hereinafter referred to as ITO), indium zinc oxide (hereinafter, also
referred to as IZO), or indium tin oxide to which silicon oxide is added
can be used.

[0168] Alternatively, the light-transmitting conductive film can be formed
using a conductive composition including a conductive high molecule (also
referred to as a conductive polymer). The pixel electrode formed using a
conductive composition preferably has a sheet resistance of 10000
Ω/square or less and a light transmittance of 70% or more at a
wavelength of 550 nm. Further, the resistivity of the conductive high
molecule included in the conductive composition is preferably 0.1
Ωcm or less.

[0169] In this embodiment, an 80-nm-thick ITO film is formed as the
light-transmitting conductive film, and then the light-transmitting
conductive film is selectively etched using a resist mask formed by a
ninth photolithography step, whereby the electrode 212 which serves as a
pixel electrode is formed (see FIG. 5c).

[0170] Although the gate insulating layer 204 and the oxide semiconductor
layer 205 are successively formed in this embodiment, the formed gate
insulating layer 204 may be exposed to air, and then the oxide
semiconductor layer 205 may be formed. In that case, the gate insulating
layer 204 is preferably subjected to heat treatment (at higher than or
equal to 400° C. and lower than the strain point of the substrate)
in an atmosphere of an inert gas (such as nitrogen, helium, neon, or
argon). By this heat treatment, impurities such as hydrogen and water
included in the gate insulating layer 204 can be removed before the
formation of the oxide semiconductor film.

[0171] A silicon oxide layer, a silicon nitride layer, a silicon
oxynitride layer, or a silicon nitride oxide layer may be formed by a
plasma CVD method instead of a sputtering method. For example, a silicon
oxynitride layer may be formed by a plasma CVD method using SiH4,
oxygen, and nitrogen as a deposition gas. The thickness of the gate
insulating layer 204 is greater than or equal to 100 nm and less than or
equal to 500 nm. In the case of a stacked-layer structure, a first gate
insulating layer with a thickness of greater than or equal to 50 nm and
less than or equal to 200 nm and a second gate insulating layer with a
thickness of greater than or equal to 5 nm and less than or equal to 300
nm are stacked in this order. When the film formed by a plasma CVD method
or the like includes an impurity such as hydrogen or water, the above
heat treatment is preferably performed so that the impurity is removed,
and then the oxide semiconductor film is formed.

[0172] Although the gate insulating layer is selectively etched for
formation of the contact hole which reaches a gate wiring layer (not
illustrated) through the fourth photolithography step in this embodiment,
an embodiment of the present invention is not limited to this method. For
example, after the gate insulating layer 204 is formed, a resist mask is
formed over the gate insulating layer and the contact hole reaching the
gate wiring layer may be formed.

[0173] After the formation of the oxide semiconductor layer 205, the oxide
semiconductor layer 205 may be subjected to dehydration or
dehydrogenation.

[0174] First heat treatment for dehydration or dehydrogenation is
performed at a temperature higher than or equal to 400° C. and
lower than 750° C., preferably 425° C. or higher. Note that
the heat treatment may be performed for 1 hour or shorter when the
temperature thereof is higher than or equal to 425° C., although
the heat treatment is performed for longer than 1 hour when the
temperature is lower than 425° C. In the first heat treatment, the
substrate is introduced into an electric furnace, which is one of heat
treatment apparatuses, and heat treatment is performed on the oxide
semiconductor layer in a nitrogen atmosphere. Then, the oxide
semiconductor layer is not exposed to air, which prevents water and
hydrogen from entering the oxide semiconductor layer again; thus, the
oxide semiconductor layer is obtained. The same furnace is used from the
heating temperature T at which the oxide semiconductor layer is subjected
to dehydration or dehydrogenation to a temperature sufficient to prevent
water from entering again; specifically, slow cooling is performed in a
nitrogen atmosphere until the temperature drops by 100° C. or more
from the heating temperature T. Furthermore, without limitation to a
nitrogen atmosphere, dehydration or dehydrogenation is performed in an
atmosphere of a rare gas such as helium, neon, or argon.

[0175] Note that the heat treatment apparatus is not limited to the
electric furnace; for example, a rapid thermal anneal (RTA) apparatus
such as a gas rapid thermal anneal (GRTA) apparatus or a lamp rapid
thermal anneal (LRTA) apparatus can be used. An LRTA apparatus is an
apparatus for heating an object to be processed by radiation of light (an
electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal
halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium
lamp, or a high pressure mercury lamp. A GRTA apparatus is an apparatus
for heating an object to be processed by thermal radiation of light
emitted from the above lamp and by conduction of heat from a gas heated
by light emitted from a lamp. As the gas, an inert gas which does not
react with an object to be processed by heat treatment, such as nitrogen
or a rare gas like argon is used. An LRTA apparatus or a GRTA apparatus
may be provided with not only a lamp but also a device for heating an
object to be processed by heat conduction or heat radiation from a heater
such as a resistance heater.

[0176] In the first heat treatment, it is preferable that water, hydrogen,
and the like be not included in nitrogen or a rare gas such as helium,
neon, or argon. It is preferable that the purity of nitrogen or a rare
gas such as helium, neon, or argon which is introduced into a heat
treatment apparatus be 6N (99.9999%) or higher, preferably 7N (99.99999%)
or higher (that is, the impurity concentration is 1 ppm or lower,
preferably 0.1 ppm or lower).

[0177] Note that the oxide semiconductor layer may be crystallized to be a
microcrystalline film or a polycrystalline film, depending on the
condition of the first heat treatment or the material of the oxide
semiconductor layer. For example, the oxide semiconductor layer may be
crystallized to be a microcrystalline oxide semiconductor film having a
degree of crystallization of 90% or higher, or 80% or higher. Further,
depending on the condition of the first heat treatment or the material of
the oxide semiconductor layer, the oxide semiconductor layer may be an
amorphous oxide semiconductor film including no crystalline component.

[0178] The oxide semiconductor layer is changed to an oxygen-deficient and
low-resistance oxide semiconductor layer after the first heat treatment.
The carrier concentration of the oxide semiconductor film after the first
heat treatment is higher than that of the oxide semiconductor film just
after the deposition; it is preferable that the oxide semiconductor layer
have a carrier concentration of 1×1018/cm3 or higher.

[0179] The first heat treatment for the oxide semiconductor layer can be
performed before the oxide semiconductor film is processed into the
island-shaped oxide semiconductor layer. In that case, after the first
heat treatment, the substrate is taken out of the heating apparatus and
the third photolithography step is performed.

[0180] After the insulating layer 208 is formed, second heat treatment
(preferably at higher than or equal to 200° C. and lower than or
equal to 400° C., for example, at higher than or equal to
250° C. and lower than or equal to 350° C.) may be
performed in an inert gas atmosphere or in a nitrogen gas atmosphere.

[0181] For example, the second heat treatment is performed in a nitrogen
atmosphere at 250° C. for 1 hour. In the second heat treatment,
heating is performed in a state where part of the oxide semiconductor
layer 205 is in contact with the oxide insulating layer 208a and other
part of the oxide semiconductor layer 205 is in contact with the
electrode 207a and the electrode 207b.

[0182] When the second heat treatment is performed in a state where the
oxide semiconductor layer 205 whose resistance is lowered through the
first heat treatment is in contact with the oxide insulating layer 208a,
the region of the oxide semiconductor layer 205 in contact with the oxide
insulating layer 208a is brought into an oxygen-excess state. As a
result, the oxide semiconductor layer 205 is changed into a
high-resistance (i-type) oxide semiconductor layer in the depth direction
from the region in contact with the oxide insulating layer 208a.

[0183] Specifically, in the oxide semiconductor layer 205, a
high-resistance (i-type) region is formed from the interface where the
oxide semiconductor layer 205 is in contact with the oxide insulating
layer 208a to the insulating layer 204.

[0184] Since the high-resistance (i-type) oxide semiconductor layer is
formed in the channel formation region of the thin film transistor
manufactured in this embodiment, the threshold voltage is a positive
value and the thin film transistor behaves as an enhancement-type thin
film transistor.

[0185] When the second heat treatment is performed on the region where the
oxide semiconductor layer 205 is in contact with the electrode 207a and
the electrode 207b which are formed using the metal conductive film with
high oxygen affinity, oxygen easily moves to the metal conductive film
side and the oxide semiconductor layer in the region in contact with the
metal conductive film with high oxygen affinity is changed into an
n-type. As an example of metal with high oxygen affinity, Ti can be
given.

[0186] The timing of the second heat treatment is not limited to
immediately after the sixth photolithography step as long as it is after
the sixth photolithography step.

[0187] Note that a photolithography step using a multi-tone mask can also
be applied to this embodiment. A photolithography step using a multi-tone
mask will be described with reference to FIGS. 6A to 6D and FIGS. 7A to
7E.

[0188] A multi-tone mask is a photomask which can perform three levels of
light exposure to obtain an exposed portion, a half-exposed portion, and
an unexposed portion. Light has a plurality of intensities after passing
through a multi-tone mask. One-time light exposure and development
process with a multi-tone mask can form a resist mask with regions of
plural thicknesses (typically, two kinds of thicknesses). Accordingly, by
using a multi-tone mask, the number of photomasks can be reduced.

[0189] As typical examples of the multi-tone mask, a gray-tone mask 801a
illustrated in FIG. 6A and a half-tone mask 801b illustrated in FIG. 6C
are given.

[0190] As illustrated in FIG. 6A, the gray-tone mask 801a includes a
light-transmitting substrate 802, and a light-blocking portion 803 and a
diffraction grating 804 which are formed on the light-transmitting
substrate 802. The light transmittance of the light-blocking portion 803
is 0%. On the other hand, the diffraction grating 804 has a
light-transmitting portion in a slit form, a dot form, a mesh form, or
the like with intervals which are equal to or less than the resolution
limit of light used for the light exposure; thus, the light transmittance
can be controlled. The diffraction grating 804 can have
regularly-arranged slits, dots, or meshes, or irregularly-arranged slits,
dots, or meshes.

[0191] As the light-transmitting substrate 802, a light-transmitting
substrate such as a quartz substrate can be used. The light-blocking
portion 803 and the diffraction grating 804 can be formed using a
light-blocking material which absorbs light, such as chromium or chromium
oxide.

[0192] When the gray-tone mask 801a is irradiated with light for exposure,
a light transmittance 805 of the light-blocking portion 803 is 0% and the
light transmittance 805 of a region where neither the light-blocking
portion 803 nor the diffraction grating 804 is provided is 100% as shown
in FIG. 6B. The light transmittance 805 of a region where the diffraction
grating 804 is provided can be controlled in the range of 10% to 70%. The
light transmittance of the diffraction grating 804 can be controlled by
adjusting the interval and pitch of slits, dots, or meshes of the
diffraction grating.

[0193] As shown in FIG. 6C, the half-tone mask 801b includes the
light-transmitting substrate 802, and a semi-light-transmitting portion
807 and a light-blocking portion 806 which are formed on the
light-transmitting substrate 802. The semi-light-transmitting portion 807
can be formed using MoSiN, MoSi, MoSiO, MoSiON, CrSi, or the like. The
light-blocking portion 806 can be formed using a light-blocking material
which absorbs light, such as chromium or chromium oxide.

[0194] When the half-tone mask 801b is irradiated with light for exposure,
a light transmittance 808 of the light-blocking portion 806 is 0% and the
light transmittance 808 of a region where neither the light-blocking
portion 806 nor the semi-light-transmitting portion 807 is provided is
100% as shown in FIG. 6D. The light transmittance 808 of a region where
the semi-light-transmitting portion 807 is provided can be controlled in
the range of 10% to 70%. The light transmittance of the
semi-light-transmitting portion 807 can be controlled by a material of
the semi-light-transmitting portion 807.

[0195] Next, with reference to FIGS. 7A to 7E, an example in which the
third photolithography step and the fifth photolithography step are
replaced with one photolithography step using a multi-tone mask will be
described.

[0196] In the third photolithography step of this embodiment, the
semiconductor layer 205 is formed over the insulating layer 204, and is
then processed into an island-shaped semiconductor layer. In this
example, however, the semiconductor layer 205 is not processed into an
island-shaped semiconductor layer and an electrode layer 207 is formed
over the semiconductor layer 205 successively to the formation thereof.
Then, a resist mask 231 having a depressed portion and a projected
portion is formed over the electrode layer 207 using a multi-tone mask
(see FIG. 7A).

[0197] The resist mask 231 can also be referred to as a resist mask
including a plurality of regions (here, two regions) having different
thicknesses. In the resist mask 231, a thick region is called a projected
portion of the resist mask 231, and a thin region is called a depressed
portion of the resist mask 231.

[0198] In the resist mask 231, a projected portion is formed in a region
where the electrode 207a serving as a source electrode and the electrode
207b serving as a drain electrode are formed and a depressed portion is
formed in a region between the electrode 207a and the electrode 207b.

[0199] Then, the electrode layer 207 and the semiconductor layer 205 are
selectively etched at the same time using the resist mask 231 to form the
semiconductor layer 205 having an island shape (see FIG. 7B).

[0200] Then, the resist mask 231 is reduced (downsized) to form resist
masks 231a and 231b. In order to reduce (downsize) the resist mask,
oxygen plasma ashing or the like may be performed. When the resist mask
is reduced (downsized), part of the electrode layer 207 which is
sandwiched between the resist masks 231a and 231b is exposed (see FIG.
7C).

[0201] Then, part of the electrode layer 207 which is sandwiched between
the resist masks 231a and 231b are selectively etched using the resist
masks 231a and 231b to provide the electrode 207a and the electrode 207b.
Note that the semiconductor layer 205 is partly etched at this time to be
a semiconductor layer having a groove (a depressed portion). In addition,
edges of the semiconductor layer 205 extend beyond edges of the electrode
207a and the electrode 207b (see FIG. 7D). Then, the resist masks 231a
and 231b are removed (see FIG. 7E).

[0202] By using the multi-tone mask, a plurality of photolithography steps
can be replaced with one photolithography step. Accordingly, the
productivity of a semiconductor device can be improved.

[0203] In this embodiment, a thin film transistor 253 may be formed to
have the following structure. At the time of forming the contact hole for
a connection between the electrode 207a and the source wiring 209 by the
sixth photolithography step, an opening is formed in the insulating layer
204b, the insulating layer 208a, and the insulating layer 208b so as to
surround the thin film transistor, and the insulating layer 211 is in
contact with the insulating layer 204a through the opening. An example of
a cross-sectional view of the thin film transistor 253 is illustrated in
FIG. 38.

[0204] The thin film transistor 253 illustrated in FIG. 38 is a
channel-etched thin film transistor like the thin film transistor 250 and
includes the insulating layer 201 provided over the substrate 200, the
gate wiring 202 provided over the insulating layer 201, the gate wiring
203 provided over the gate wiring 202, the insulating layer 204a provided
over the gate wiring 203, the insulating layer 204b provided over the
insulating layer 204a, the semiconductor layer 205 provided over the
insulating layer 204b, the pair of electrodes 207a and 207b provided over
the semiconductor layer 205, the insulating layer 208a provided over the
electrode 207a, the electrode 207b, and the semiconductor layer 205, the
insulating layer 208b provided over the insulating layer 208a, the source
wiring 209 which is in contact with the electrode 207a through an opening
provided in the insulating layer 208a and the insulating layer 208b, the
source wiring 210 provided over the source wiring 209, the insulating
layer 211 provided over the source wiring 210, and the electrode 212
which is in contact with the electrode 207b through an opening provided
in the insulating layer 211, the insulating layer 208a, and the
insulating layer 208b.

[0205] Here, in the insulating layer 204b, the insulating layer 208a, and
the insulating layer 208b, an opening is selectively formed by the sixth
photolithography step so that the insulating layer 204a is exposed, and
the insulating layer 211 covers a top surface and a side surface of the
insulating layer 208b and side surfaces of the insulating layer 208a and
the insulating layer 204b and is in contact with the insulating layer
204a through the opening.

[0206] Here, the insulating layer 211 and the insulating layer 204a are
formed using insulating films including nitrogen and are inorganic
insulating films which do not include an impurity such as moisture, a
hydrogen ion, or OH.sup.- and block entry of these from the outside.

[0207] Thus, with the structure illustrated in FIG. 38, the thin film
transistor 253 can be hermetically sealed with the insulating layer 211
and the insulating layer 204a which are formed using the insulating films
including nitrogen; consequently, entry of moisture from the outside can
be prevented in a manufacturing process after formation of the insulating
layer 211. Further, even after a device is completed as a display device
such as a liquid crystal display device, entry of moisture from the
outside can be prevented in the long term; therefore, long-term
reliability of the device can be improved.

[0208] In this embodiment, the structure in which one thin film transistor
is surrounded by insulating films including nitrogen is described;
however, an embodiment of the present invention is not particularly
limited thereto. A plurality of thin film transistors may be surrounded
by insulating films including nitrogen, or a plurality of thin film
transistors in a pixel portion may be collectively surrounded by
insulating films including nitrogen. A region where the insulating layer
211 and the insulating layer 204a are in contact with each other may be
formed so that at least the periphery of the pixel portion of the active
matrix substrate is surrounded.

[0209] Further, a conductive film mainly including Al as a wiring material
with low electric resistance can be used for the gate wiring 203 and the
source wiring 209. An example of a pixel structure of the display device
illustrated in FIGS. 1A and 1B is described with reference to FIGS. 39A
to 39C. FIG. 39A is a top view illustrating a plan structure of the
pixel, and FIGS. 39B and 39C are cross-sectional views each illustrating
a stacked-layer structure of the pixel. Note that chain lines A1-A2,
B1-B2, and C1-C2 in FIG. 39A correspond to cross sections A1-A2, B1-B2,
and C1-C2 in FIG. 39B, respectively. Chain line D1-D2 in FIG. 39A
corresponds to cross section D1-D2 in FIG. 39C.

[0210] In cross section A1-A2 and cross section D1-D2, stacked-layer
structures of a thin film transistor 252 used in the pixel portion are
illustrated. The thin film transistor 252 is one embodiment of a thin
film transistor having a bottom gate structure.

[0211] In cross section A1-A2 and cross section D1-D2, an insulating layer
201 provided over a substrate 200, a gate wiring 203 provided over the
insulating layer 201, an insulating layer 204 provided over the gate
wiring 203, a semiconductor layer 205 provided over the insulating layer
204, a pair of electrodes 207a and 207b provided over the semiconductor
layer 205, an insulating layer 208 provided over the electrode 207a, the
electrode 207b, and the semiconductor layer 205, a source wiring 209
which is in contact with the electrode 207a through an opening provided
in the insulating layer 208, an insulating layer 211 provided over the
source wiring 209, and an electrode 212 which is in contact with the
electrode 207b through an opening provided in the insulating layer 211
and the insulating layer 208 are illustrated.

[0212] Further, in cross section B1-B2, a stacked-layer structure of a
storage capacitor (also referred to as a Cs capacitor) is illustrated. In
cross section B1-B2, the insulating layer 201 over the substrate 200, a
storage capacitor wiring 213 over the insulating layer 201, the
insulating layer 204 over the storage capacitor wiring 213, the electrode
207b over the insulating layer 204, the insulating layer 208 over the
electrode 207b, the insulating layer 211 over the insulating layer 208,
and the electrode 212 over the insulating layer 211 are illustrated.

[0213] Further, in cross section C1-C1, a stacked-layer structure in a
wiring intersection of the gate wiring and the source wiring is
illustrated. In cross section C1-C2, the insulating layer 201 over the
substrate 200, the gate wiring 203 over the insulating layer 201, the
insulating layer 204 over the gate wiring 203, the insulating layer 208
over the insulating layer 204, the source wiring 209 over the insulating
layer 208, and the insulating layer 211 over the source wiring 209 are
illustrated. Note that a semiconductor layer may be formed between the
insulating layer 204 and the insulating layer 208 in the wiring
intersection.

[0214] For the conductive film mainly including Al as a wiring material
with low electric resistance, pure aluminum can be used; however, it is
preferable to use an aluminum alloy including an element which improves
heat resistance or an element which prevents hillocks, such as titanium
(Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr),
neodymium (Nd), scandium (Sc), nickel (Ni), platinum (Pt), copper (Cu),
gold (Au), silver (Ag), manganese (Mn), carbon (C), or silicon (Si), or
an alloy material or a compound including any of the elements as a main
component. Alternatively, a stack of a conductive film mainly including
Al and a conductive film including an element with a higher melting point
than Al, such as W, Ta, Mo, Ti, or Cr, can be used.

[0215] In the example of the pixel structure of the display device
illustrated in FIGS. 39A to 39C, a conductive film having three-layer
structure in which a molybdenum film, a conductive film mainly including
Al, and a molybdenum film are stacked in this order, or a conductive film
in which titanium and aluminum are stacked may be used.

[0216] A gate electrode is formed in an early stage of the process of a
bottom gate transistor. Therefore, a material applied to the gate
electrode needs to be selected in consideration of resistance to heat
treatment in a process subsequent to the formation of the gate electrode.
However, heat treatment in the process described in this embodiment is
performed under mild conditions; therefore, the range of materials that
can be applied to the gate electrode is wide and the material can be
selected in accordance with performance required for the display device.
For example, although a conductive film mainly including Al is useful
because of its relatively low electric resistance, inexpensiveness, and
excellent processability, its heat resistance is relatively low.
Therefore, according to the process of this embodiment, even the
conductive film mainly including Al can be used for the gate electrode.

[0217] Further, a light-transmitting thin film transistor can be provided.
Here, the case where, in the thin film transistor provided in the pixel
portion of the display device described in Embodiments 1 and 2, the
light-transmitting oxide semiconductor layer 205 is used and
light-transmitting conductive films are applied to the gate wiring 203,
the electrode 207a, and the electrode 207b is described.

[0218] When the thin film transistor included in the pixel structure
illustrated in FIG. 2B is a light-transmitting thin film transistor, a
light-transmitting thin film transistor including the insulating layer
201 provided over the substrate 200, the gate wiring 202 provided over
the insulating layer 201, the light-transmitting gate wiring 203 provided
over the gate wiring 202, the insulating layer 204 provided over the gate
wiring 203, the semiconductor layer 205 provided over the insulating
layer 204, the pair of light-transmitting electrodes 207a and 207b
provided over the semiconductor layer 205, the insulating layer 208
provided over the electrode 207a, the electrode 207b, and the
semiconductor layer 205, the source wiring 209 which is in contact with
the electrode 207a through an opening provided in the insulating layer
208, the source wiring 210 provided over the source wiring 209, the
insulating layer 211 provided over the source wiring 210, and the
electrode 212 which is in contact with the electrode 207b through an
opening provided in the insulating layer 211 and the insulating layer 208
can be provided.

[0219] Alternatively, a bottom gate thin film transistor including a
channel protective layer, which is illustrated as an example in FIG. 3,
may be a light-transmitting thin film transistor. Specifically, a
light-transmitting thin film transistor including the insulating layer
201 provided over the substrate 200, the gate wiring 202 provided over
the insulating layer 201, the light-transmitting gate wiring 203 provided
over the gate wiring 202, the insulating layer 204 provided over the
light-transmitting gate wiring 203, the semiconductor layer 205 provided
over the insulating layer 204, the channel protective layer 225 provided
over the semiconductor layer 205, the pair of light-transmitting
electrodes 207a and 207b provided over the channel protective layer 225,
the insulating layer 208 provided over the electrode 207a, the electrode
207b, and the semiconductor layer 205, the source wiring 209 which is in
contact with the electrode 207a through an opening provided in the
insulating layer 208, the source wiring 210 provided over the source
wiring 209, the insulating layer 211 provided over the source wiring 210,
and the electrode 212 which is in contact with the electrode 207b through
an opening provided in the insulating layer 211 and the insulating layer
208 can be provided.

[0220] Most of the oxide semiconductors that can be applied to the oxide
semiconductor layer 205 described in Embodiment 2 transmits visible
light. A film including a light-transmitting conductive material, for
example, indium oxide including tungsten oxide, indium zinc oxide
including tungsten oxide, indium oxide including titanium oxide, indium
tin oxide including titanium oxide, indium tin oxide (hereinafter
referred to as ITO), indium zinc oxide, indium tin oxide to which silicon
oxide is added, an In--Sn--Zn--O-based oxide semiconductor, an
In--Al--Zn--O-based oxide semiconductor, a Sn--Ga--Zn--O-based oxide
semiconductor, an Al--Ga--Zn--O-based oxide semiconductor, a
Sn--Al--Zn--O-based oxide semiconductor, a Sn--Zn--O-based oxide
semiconductor, an Al--Zn--O-based oxide semiconductor, an In--O-based
oxide semiconductor, a Sn--O-based oxide semiconductor, a Zn--O-based
oxide semiconductor, or the like can be formed by a sputtering method or
the like and applied to the gate wiring 203, the electrode 207a, and the
electrode 207b.

[0221] Since the thin film transistor where the light-transmitting oxide
semiconductor layer 205 is used and light-transmitting conductive films
are applied to the gate wiring 203, the electrode 207a, and the electrode
207b has a light-transmitting property, the aperture ratio in the pixel
portion is not reduced.

[0222] Note that a light-transmitting conductive oxide functions as an
n.sup.+ layer in a region in contact with the oxide semiconductor layer;
therefore, the thin film transistor can have low contact resistance and
parasitic resistance.

[0223] Through the above process, a semiconductor device typified by a
display device having high display quality, in which increase in wiring
resistance is suppressed, can be provided. Moreover, a highly reliable
semiconductor device can be provided in which insulating layers including
silicon nitride are formed as insulating layers located over and under a
conductive layer including Cu so that the conductive layer including Cu
is sandwiched between or surrounded by the insulating layers, whereby Cu
diffusion is prevented.

[0224] When an oxide semiconductor layer which is formed by the method
described in this embodiment as an example and whose impurity
concentration is suppressed is used, a highly reliable semiconductor
element can be provided. Specifically, a thin film transistor including
an oxide semiconductor, whose threshold voltage is controlled, can be
provided. Moreover, a thin film transistor including an oxide
semiconductor, which has high operation speed and sufficient reliability
and can be manufactured through a relatively simple process, can be
provided.

[0225] Furthermore, according to this embodiment, a method for
manufacturing a thin film transistor including an oxide semiconductor,
whose threshold voltage is controlled and which has high operation speed
and sufficient reliability and can be manufactured through a relatively
simple process, can be provided.

[0226] Note that this embodiment can be combined with any of the other
embodiments in this specification as appropriate.

Embodiment 3

[0227] In this embodiment, an example of a structure of a thin film
transistor used for the gate driver circuit 91 or the source driver
circuit 92 of the display device 30 which is described in Embodiment 1
with reference to FIGS. 1A and 1B will be described.

[0228] A driver circuit for driving a pixel portion is formed using an
inverter circuit, a capacitor, a resistor, and the like. In this
embodiment, a structure of an inverter circuit which includes two thin
film transistors as thin film transistors used in the driver circuit is
described. When the inverter circuit is formed using two n-channel TFTs
in combination, there are an inverter circuit having two enhancement type
TFTs (hereinafter referred to as an EEMOS circuit) and an inverter
circuit having a combination of an enhancement type transistor and a
depletion type transistor (hereinafter referred to as an EDMOS circuit).
Note that an n-channel TFT whose threshold voltage is positive is
referred to as an enhancement type transistor, and an n-channel TFT whose
threshold voltage is negative is referred to as a depletion type
transistor, throughout this specification.

[0229]FIG. 8A illustrates a cross-sectional structure of the inverter
circuit of the driver circuit. Further, FIG. 8c is a top view of the
inverter circuit of the driver circuit. A cross section taken along chain
line Z1-Z2 in FIG. 8c corresponds to FIG. 8A. Note that a first thin film
transistor 430a and a second thin film transistor 430b illustrated in
FIGS. 8A to 8C are inverted staggered thin film transistors having a
bottom gate structure.

[0230] In the first thin film transistor 430a illustrated in FIG. 8A, a
first gate wiring 401a is provided over a substrate 400 provided with an
insulating layer 410, an insulating layer 411 and an insulating layer 412
are provided over the first gate wiring 401a, a first semiconductor layer
403a is provided over the insulating layer 412, and an electrode 405a and
an electrode 405b are provided over the first semiconductor layer 403a.
In a similar manner, in the second thin film transistor 430b, a second
gate wiring 401b is provided over the substrate 400 provided with the
insulating layer 410, the insulating layer 411 and the insulating layer
412 are provided over the second gate wiring 401b, a second semiconductor
layer 403b is provided over the insulating layer 412, and the electrode
405b and an electrode 405c are provided over the second semiconductor
layer 403b. Here, the electrode 405c is directly connected to the second
gate wiring 401b through a contact hole 404 formed in the insulating
layer 411 and the insulating layer 412. Further, an insulating layer 413,
an insulating layer 414, and an insulating layer 415 are formed over the
electrode 405a, the electrode 405b, and the electrode 405c. Note that the
electrodes 405a to 405c are extended as illustrated in FIG. 8c, and also
function as wirings which electrically connect the thin film transistors
in the driver circuit.

[0231] Here, the first gate wiring 401a and the second gate wiring 401b
can be formed using a material and a method similar to those of the gate
wiring 203 described in Embodiment 1 or 2. The first semiconductor layer
403a and the second semiconductor layer 403b can be formed using a
material and a method similar to those of the semiconductor layer 205
described in Embodiment 1 or 2. The electrode 405a, the electrode 405b,
and the electrode 405c can be formed using a material and a method
similar to those of the pair of electrodes 207a and 207b described in
Embodiment 1 or 2. The insulating layers 410 to 415 can be formed using
materials and methods similar to those of the insulating layer 201, the
insulating layers 204a and 204b, the insulating layers 208a and 208b, and
the insulating layer 211, which are described in Embodiment 1 or 2,
respectively.

[0232] Further, the contact hole 404 is formed in such a manner that, by
the fourth photolithography step described in Embodiment 2, a mask is
formed over the insulating layer 412 and used for selective etching of
the insulating layer 412 and the insulating layer 411. By the direct
connection between the electrode 405c and the second gate wiring 401b
through the contact hole 404, favorable contact can be obtained, which
leads to reduction in contact resistance. Moreover, as compared to the
case where the electrode 405c is connected to the second gate wiring 401b
through another conductive film such as a transparent conductive film,
the number of contact holes can be reduced; consequently, the area
occupied by the thin film transistor can be reduced and the distance
between thin film transistors in the driver circuit can be shortened.

[0233] As described above, the distance between the thin film transistors
in the driver circuit can be shortened and wiring resistance can be
sufficiently reduced; therefore, a conductive layer including Cu is not
necessarily used as a wiring which is electrically connected to a thin
film transistor. Accordingly, the distance between the thin film
transistor in the driver circuit and a conductive layer including Cu can
be long enough, whereby diffusion of Cu into the oxide semiconductor
layer can be prevented. However, a power supply line which supplies a
power supply potential to each thin film transistor or a wiring such as a
common wiring which is relatively long is affected by wiring resistance
relatively easily. Thus, a wiring formed using the conductive layer
including Cu is preferably used for such wirings.

[0234] In addition, as described in Embodiment 2, since heat treatment in
the process described in Embodiment 3 is performed under mild conditions,
the range of materials that can be applied to the gate electrode is wide
and a material can be selected in accordance with performance required
for the display device. For example, although a conductive film mainly
including Al is useful because of its relatively low electric resistance,
inexpensiveness, and excellent processability, its heat resistance is
relatively low. However, according to the process of this embodiment,
even the conductive film mainly including Al can be used for the gate
electrode.

[0235] As described in Embodiment 1, the gate driver circuit 91 is
connected to the gate wirings (20_1 to 20--n (note that n is a
natural number)), and the source driver circuit 92 is connected to the
source wirings (60_1 to 60--m (note that m is a natural number));
the gate wirings (20_1 to 20--n (note that n is a natural number))
and the source wirings (60_1 to 60--m (note that m is a natural
number)) are formed using the conductive layer including Cu. Therefore,
even in a display portion where the distance led by the wirings is long,
wiring resistance can be sufficiently reduced.

[0236] The electrode 405a is a power supply line at a ground potential (a
ground power supply line). This power supply line at a ground potential
may be a power supply line to which negative voltage VDL is applied (a
negative power supply line). The electrode 405c is electrically connected
to a power supply line to which positive voltage VDD is applied (a
positive power supply line).

[0237] An equivalent circuit of the EEMOS circuit is illustrated in FIG.
8B. The circuit connection illustrated in FIGS. 8A and 8C corresponds to
the equivalent circuit illustrated in FIG. 8B, and the first thin film
transistor 430a and the second thin film transistor 430b are enhancement
type n-channel transistors as an example.

[0238] Note that gate electrodes may be provided above and below oxide
semiconductor layers and the threshold voltage may be controlled so that
the first thin film transistor 430a and the second thin film transistor
430b may behave as enhancement type n-channel transistors.

[0239] Further, not being limited to the EEMOS circuit, an EDMOS circuit
can also be manufactured by forming the first thin film transistor 430a
to be an enhancement type n-channel transistor and forming the second
thin film transistor 430b to be a depletion type n-channel transistor. In
that case, instead of the electrode 405c, the electrode 405b is connected
to the second gate wiring 401b.

[0240] In order to manufacture an enhancement type n-channel transistor
and a depletion type n-channel transistor over one substrate, for
example, the first semiconductor layer 403a and the second semiconductor
layer 403b are formed using different materials or under different
conditions. An EDMOS circuit may be formed in such a manner that gate
electrodes for controlling the threshold value are provided over oxide
semiconductor layers to control the threshold value and voltage is
applied to the gate electrodes for controlling the threshold value so
that one of the TFTs is normally on while the other TFT is normally off.

[0241] Note that the structure described in this embodiment can be
combined with any of the structures described in the other embodiments as
appropriate.

Embodiment 4

[0242] In this embodiment, a protection circuit using a semiconductor
element will be described with reference to FIGS. 9A and 9B and FIG. 10.
In addition, a structure of a connection portion where different common
wirings between which an insulating film is provided are connected will
be described with reference to FIGS. 11A and 11B.

[0243] An example of a circuit that can be applied to the protection
circuit 97 is illustrated in FIG. 9A. This protection circuit includes
non-linear elements 170a and 170b. Each of the non-linear elements 170a
and 170b includes a two-terminal element such as a diode or a
three-terminal element such as a transistor. For example, the non-linear
element can be formed through the same process as the transistor in the
pixel portion. For example, characteristics similar to those of a diode
can be obtained by connecting a gate terminal to a drain terminal of the
non-linear element.

[0244] A first terminal (gate) and a third terminal (drain) of the
non-linear element 170a are connected to the common wiring 45, and a
second terminal (source) thereof is connected to the source wiring 60_1.
A first terminal (gate) and a third terminal (drain) of the non-linear
element 170b are connected to the source wiring 60_1, and a second
terminal (source) thereof is connected to the common wiring 45. That is,
the protection circuit illustrated in FIG. 9A has a structure in which
the two transistors are each connected to the common wiring 45 and the
source wiring 60_1 so as to have opposite rectifying directions. In other
words, a transistor whose rectifying direction is from the common wiring
45 to the source wiring 60_1 and a transistor whose rectifying direction
is from the source wiring 60_1 to the common wiring 45 are connected
between the common wiring 45 and the source wiring 60_1.

[0245] In the above protection circuit, when the source wiring 60_1 is
positively or negatively charged due to static electricity or the like,
current flows in a direction that cancels the charge. For example, when
the source wiring 60_1 is positively charged, current flows in a
direction in which the positive charge is released to the common wiring
45. Owing to this operation, electrostatic breakdown or a shift in the
threshold voltage of a pixel transistor connected to the charged source
wiring 60_1 can be prevented. Moreover, it is possible to prevent
dielectric breakdown of an insulating layer, between the charged source
wiring 60_1 and another wiring that intersect with each other with the
insulating layer interposed therebetween.

[0246] It is to be noted that the protection circuit is not limited to the
above structure. For example, a structure in which a plurality of
transistors whose rectifying direction is from the common wiring 45 to
the source wiring 60_1 and a plurality of transistors whose rectifying
direction is from the source wiring 60_1 to the common wiring 45 are
connected may be employed. By connecting the common wiring 45 and the
source wiring 60_1 with a plurality of non-linear elements, charges can
be prevented from being directly applied to the source wiring 60_1 not
only in the case where surge voltage is applied to the source wiring 60_1
but also in the case where the common wiring 45 is charged due to static
electricity or the like. In addition, a protection circuit can be
configured using an odd number of non-linear elements.

[0247] Although FIG. 9A illustrates an example in which the protection
circuit is provided for the source wiring 60_1 and the common wiring 45,
a similar configuration can be applied to a protection circuit of another
portion. Note that the protection circuit of FIG. 9A can be formed by
applying a semiconductor element of one embodiment of the present
invention to the non-linear element 170a and the non-linear element 170b.

[0248] Next, an example in which a protection circuit is formed over a
substrate with the use of a semiconductor element of one embodiment of
the present invention is described with reference to FIG. 9B and FIG. 10.
Note that FIG. 9B is an example of a top view of wirings and a connection
portion between the wirings, and FIG. 10 is a cross-sectional view taken
along line Q1-Q2, line Q3-Q4, and line Q5-Q6 in FIG. 9B.

[0249]FIG. 9B is a top view of a portion where the common wiring 45 and
the source wiring 60_1 are connected with the non-linear element 170a and
the non-linear element 170b and illustrates an example of a structure of
the protection circuit 97.

[0250] The non-linear element 170a includes a gate wiring 111a, and the
gate wiring 111a is connected to the common wiring 45. One of a source
electrode and a drain electrode of the non-linear element 170a is
connected to the source wiring 60_1, and the other thereof is formed of a
first electrode 115a. Further, the first electrode 115a is connected to
the common wiring 45.

[0251] The non-linear element 170b includes a gate wiring 111b, and the
gate wiring 111b is connected to the source wiring 60_1 through a contact
hole 126, a second electrode 115b, and a contact hole 125. A source
electrode and a drain electrode of the non-linear element 170b are formed
of the first electrode 115a and the second electrode 115b. Further, the
non-linear element 170b includes a semiconductor layer 113.

[0252] Next, structures of the common wiring 45, the source wiring 60_1,
and the non-linear element 170b are described with reference to FIG. 10.

[0253] The common wiring 45 is formed using the same wiring layer as the
gate wiring. The common wiring 45 is formed so that a gate wiring 45a and
a gate wiring 45b are stacked over an insulating film 101 which is
provided over the substrate 100. Note that an insulating layer 102 is
formed over the gate wiring 45b, an insulating layer 117 is provided over
the insulating layer 102, and an insulating layer 118 is formed over the
insulating layer 117.

[0254] The source wiring 60_1 is formed over the insulating layer 102. The
source wiring 60_1 is formed so that a source wiring 60_1a and a source
wiring 60_1b are stacked in this order. Note that an insulating film 119
is formed over the source wiring 60_1.

[0255] The non-linear element 170b includes the gate wiring 111b over the
insulating film 101 which is provided over the substrate 100, and the
insulating layer 102 over the gate wiring 111b. Further, the non-linear
element 170b includes the semiconductor layer 113 over the gate wiring
111b with the insulating layer 102 interposed therebetween, and the
electrode 115a and the electrode 115b which are in contact with the
semiconductor layer 113 with end portions thereof overlapping with the
gate wiring 111b. The insulating layer 117 is formed to overlap with the
gate wiring 111b and be in contact with the semiconductor layer 113 which
is between the end portions of the electrode 115a and the electrode 115b,
and the insulating layer 118 is formed over the insulating layer 117.
Note that the insulating layer 102 is formed as a stack of an insulating
layer 102a and an insulating layer 102b.

[0256] The electrode 115b is directly connected to the gate wiring 111b
through the contact hole 125 provided in the insulating layer 102. The
electrode 115b is connected to the source wiring 60_1 through the contact
hole 126. The insulating film 119 is formed over the insulating layer 118
and the source wiring 60_1.

[0257] For the conductive film serving as the electrode 115a and the
electrode 115b, an element selected from Ti, Mo, W, Al, Cr, Cu, and Ta,
an alloy including any of these elements as a component, an alloy
including any of these elements in combination, or the like is used. The
conductive film is not limited to a single layer including the above
element and may be a stack of two or more layers.

[0258] Metal with oxygen affinity is particularly preferable for the
conductive film which is in contact with the semiconductor layer 113 so
that a junction of the metal with oxygen affinity and an oxide
semiconductor is formed. Titanium is particularly preferable among metals
with oxygen affinity. In this embodiment, a three-layer conductive film
in which a titanium film (with a thickness of 100 nm), an aluminum film
(with a thickness of 200 nm), and a titanium film (with a thickness of
100 nm) are stacked is formed. Instead of a titanium film, a titanium
nitride film may be used.

[0259] By providing such a junction structure between the semiconductor
layer 113 and the electrode 115a and between the semiconductor layer 113
and the electrode 115b, operation of the non-linear element 170a and the
non-linear element 170b is stabilized. That is, the thermal stability is
increased, so that stable operation becomes possible. Accordingly, the
function of the protection circuit is enhanced and the operation can be
made stable. Moreover, the amount of junction leakage is reduced, whereby
parasitic resistance in the non-linear element 170a and the non-linear
element 170b and variation in parasitic resistance can be reduced.

[0260] Note that the non-linear element 170a and the non-linear element
170b have the same structure in the main portion. The non-linear element
170b can have the same structure as the thin film transistor in the pixel
portion, which is described in Embodiment 1. Therefore, detailed
description of the non-linear element 170a and the non-linear element
170b is omitted in this embodiment. In addition, the non-linear elements
170a and 170b and the above thin film transistor can be manufactured over
one substrate through the same process.

[0261] An example of a connection between the common wirings is described
with reference to FIGS. 11A and 11B. Note that FIG. 11A is an example of
a top view of wirings and a connection portion between the wirings, and
FIG. 11B is a cross-sectional view taken along line R1-R2 and line R3-R4
in FIG. 11A.

[0262] As described above, the common wiring 45 has a structure in which
the gate wiring 45a and the gate wiring 45b are stacked in this order.
The common wiring 65 has the same structure as the source wiring 60_1.
That is, the common wiring 65 has a structure in which a source wiring
65a and a source wiring 65b are stacked in this order. The source wiring
65a is formed using the same conductive film as the source wiring 60_1a,
and the source wiring 65b is formed using the same conductive film as the
source wiring 60_1b.

[0263] In the connection portion 95, the common wiring 45 and the common
wiring 65 are electrically connected to each other. The connection
portion 95 is described with reference to FIG. 11B. The common wiring 45
and the common wiring 65 are connected to each other through a contact
hole 127 formed in the insulating layer 102, the insulating layer 117,
and the insulating layer 118.

[0264] In the connection portion 95, the gate wiring 45b and the source
wiring 65a which include a conductive material including an element with
a higher melting point than Cu are connected to each other, and thus a
highly reliable connection is realized. Furthermore, the gate wiring 45a
and the source wiring 65b which are formed using a conductive material
including Cu suppress wiring resistance.

[0265] The common connection portion 96 is provided in a region outside
the pixel portion and is a connection portion which is electrically
connected to a substrate having a connection portion that is provided to
face the common connection portion 96 through conductive particles (such
as plastic particles plated with gold). An example in which the common
connection portion 96 is formed over the conductive layer where the gate
wiring 45a and the gate wiring 45b are stacked in this order is described
with reference to FIG. 11B.

[0266] The common connection portion 96 is electrically connected to the
common wiring 45. Over the conductive layer where the gate wiring 45a and
the gate wiring 45b are stacked in this order, an electrode 115c is
formed with the insulating layer 102, the insulating layer 117, and the
insulating layer 118 interposed therebetween. The electrode 115c is
electrically connected to the conductive layer through a contact hole 128
that is formed in the insulating layer 102, the insulating layer 117, and
the insulating layer 118. A conductive layer 66 which has the same
structure as the common wiring 65 is stacked over the electrode 115c, and
then a conductive layer 129 is formed using the same transparent
conductive film as the electrode 212 which functions as a pixel
electrode.

[0267] The gate wiring 45a and the source wiring 60_1b which are connected
to the protection circuit described as an example in this embodiment are
formed using a conductive material including Cu and have low wiring
resistance.

[0268] The gate wiring 45b is formed using a conductive material including
an element with a higher melting point than Cu, such as W, Ta, Mo, Ti, or
Cr, so as to be in contact with and cover the gate wiring 45a, whereby
migration of the gate wiring 45a can be suppressed and reliability of the
semiconductor device can be improved. Further, by providing insulating
layers including silicon nitride as the insulating layers over and under
the gate wiring 45a including Cu so that the gate wiring 45a including Cu
may be sandwiched between or surrounded by the insulating layers, Cu
diffusion can be prevented.

[0269] The protection circuit described as an example in this embodiment
has a structure in which a first terminal (gate) of a non-linear element
is directly connected to a second terminal (source) or a third terminal
(drain) thereof through one contact hole. As a result, only one interface
and one contact hole are formed for one connection, which are fewer than
the numbers of interfaces and contact holes in the case of forming a
connection through another wiring layer.

[0270] Note that when the number of interfaces needed for a connection is
small, electric resistance can be suppressed. In addition, when the
number of contact holes needed for a connection is small, the area
occupied by the connection portion can be suppressed.

[0271] Accordingly, connection resistance can be suppressed in the
protection circuit described as an example in this embodiment, which
results in stable operation of the protection circuit. Moreover, since a
connection is formed using only one contact hole, the area occupied by
the protection circuit can be reduced and thus the size of the display
device can be reduced.

[0272] Note that this embodiment can be combined with any of the other
embodiments in this specification as appropriate.

Embodiment 5

[0273] In this embodiment, as for the display device described in
Embodiment 1 with reference to FIGS. 1A and 1B, examples of structures of
a gate signal line terminal in the gate terminal portion 7 and a source
signal line terminal in the source terminal portion 8 will be described.

[0274] FIGS. 12A1 and 12A2 are a cross-sectional view and a top view of
the gate signal line terminal, respectively. FIG. 12A1 is a
cross-sectional view taken along line C1-C2 in FIG. 12A2. In the gate
signal line terminal: as illustrated in FIG. 12A1, an insulating layer
360 is formed over a substrate 300; a gate wiring 351a is formed over the
insulating layer 360; a gate wiring 351b is formed to cover an end
portion of the gate wiring 351a; an insulating layer 361, an insulating
layer 362, an insulating layer 363, an insulating layer 364, and an
insulating layer 365 are formed over the gate wiring 351b; and a
transparent conductive layer 355 is formed over the insulating layer 365
and the gate wiring 351b. Here, the gate wiring 351a and the gate wiring
351b are collectively referred to as a gate wiring 351, and the gate
wiring 351b functions as a first terminal of the gate signal line
terminal. In addition, end portions of the insulating layers 361 to 365
are patterned, so that an end portion of the gate wiring 351b is exposed
and in direct contact with the transparent conductive layer 355. The
transparent conductive layer 355 which is in direct contact with the end
portion of the gate wiring 351b which is the first terminal is a
connection terminal electrode which functions as an input terminal. Here,
the gate wiring 351a, the gate wiring 351b, and the transparent
conductive layer 355 can be formed using materials and methods similar to
those of the gate wiring 202, the gate wiring 203, and the electrode 212
which are described in Embodiments 1 and 2, respectively. In addition,
the insulating layers 360 to 365 can be formed using materials and
methods similar to those of the insulating layer 201, the insulating
layers 204a and 204b, the insulating layers 208a and 208b, and the
insulating layer 211 which are described in Embodiments 1 and 2,
respectively.

[0275] By forming the gate wiring 351a with the use of a conductive
material including Cu, wiring resistance in the gate signal line terminal
and a wiring led from the gate signal line terminal can be reduced.
Further, the gate wiring 351b is formed using a conductive material
including an element with a higher melting point than Cu, such as W, Ta,
Mo, Ti, or Cr, so as to be in contact with and cover the gate wiring
351a, whereby migration of the gate wiring 351a can be suppressed and
reliability of the semiconductor device can be improved. Furthermore, the
gate wiring 351a including Cu is sandwiched between the insulating layers
360 and 361 including silicon nitride, whereby Cu diffusion from the gate
wiring 351a can be prevented.

[0276] Further, FIGS. 12B1 and 12B2 are a cross-sectional view and a top
view of the source signal line terminal, respectively. FIG. 12B1 is a
cross-sectional view taken along line D1-D2 in FIG. 12B2. In the source
signal line terminal, as illustrated in FIG. 12B1, the insulating layer
360, the insulating layer 361, and the insulating layer 362 are formed
over the substrate 300, an electrode 352 is formed over the insulating
layer 362, the insulating layer 363 and the insulating layer 364 are
formed over the electrode 352, a source wiring 354a is formed over the
insulating layer 364, a source wiring 354b is formed over the source
wiring 354a, the insulating layer 365 is formed over the source wiring
354b, and the transparent conductive layer 355 is formed over the
insulating layer 365 and the electrode 352. Here, the source wiring 354a
and the source wiring 354b are collectively referred to as a source
wiring 354. In addition, end portions of the insulating layers 363 to 365
are patterned, so that an end portion of the electrode 352 is exposed and
in direct contact with the transparent conductive layer 355. A contact
hole is formed in the insulating layer 363 and the insulating layer 364,
through which the electrode 352 functioning as a second terminal of the
source signal line terminal and the source wiring 354 are connected to
each other. The transparent conductive layer 355 which is in direct
contact with the end portion of the electrode 352 which is the second
terminal is a connection terminal electrode which functions as an input
terminal. Here, the electrode 352, the source wiring 354a, the source
wiring 354b, and the transparent conductive layer 355 can be formed using
materials and methods similar to those of the pair of electrodes 207a and
207b, the source wiring 209, the source wiring 210, and the electrode 212
which are described in Embodiments 1 and 2, respectively. Further, the
insulating layers 360 to 365 can be formed using materials and methods
similar to those of the insulating layer 201, the insulating layers 204a
and 204b, the insulating layers 208a and 208b, and the insulating layer
211 which are described in Embodiments 1 and 2, respectively.

[0277] By forming the source wiring 354b with the use of a conductive
material including Cu, wiring resistance in the source signal line
terminal and a wiring led from the source signal line terminal can be
reduced. Further, the source wiring 354a is formed using a conductive
material including an element with a higher melting point than Cu, such
as W, Ta, Mo, Ti, or Cr, an alloy including any of these elements in
combination, tantalum nitride (TaN), titanium nitride (TiN), molybdenum
nitride (MoN), or the like, so as to be in contact with the source wiring
354b, whereby migration of the source wiring 354b can be suppressed and
reliability of the semiconductor device can be improved. Furthermore, the
source wiring 354b including Cu is sandwiched between the insulating
layers 364 and 365 including silicon nitride, whereby Cu diffusion from
the source wiring 354b can be prevented.

[0278] In an example described in this embodiment, the gate wiring 351b
which is the first terminal and included in the gate wiring 351 having a
stacked-layer structure is connected to the transparent conductive layer
355 functioning as the input terminal is described in this embodiment;
however, this embodiment is not limited thereto. As illustrated in FIGS.
13A1 and 13A2, a structure in which the first terminal is constituted by
only the gate wiring 351a and the gate wiring 351a is in direct contact
with the transparent conductive layer 355 may be employed. Here, FIG.
13A1 is a cross-sectional view taken along line C1-C2 in FIG. 13A2.

[0279] Furthermore, an example in which the source wiring 354 is connected
to the transparent conductive layer 355 functioning as the input terminal
through the electrode 352 which is the second terminal is described in
this embodiment; however, this embodiment is not limited thereto. As
illustrated in FIGS. 13B1 and 13B2, in the source wiring 354 functioning
as the second terminal, the source wiring 354b may be in direct contact
with the transparent conductive layer 355. Here, FIG. 13B1 is a
cross-sectional view taken along line D1-D2 in FIG. 13B2.

[0280] A plurality of gate wirings, source wirings, and capacitor wirings
are provided depending on the pixel density. In the terminal portion, a
plurality of first terminals at the same potential as the gate wiring, a
plurality of second terminals at the same potential as the source wiring,
a plurality of third terminals at the same potential as the capacitor
wiring, and the like are arranged. The number of each of the terminals
may be any number, and the number of the terminals may be determined by a
practitioner as appropriate.

[0281] Note that the structure described in this embodiment can be
combined with any of the structures described in the other embodiments as
appropriate.

Embodiment 6

[0282] In this embodiment, an example will be described below in which at
least part of a driver circuit and a thin film transistors to be disposed
in a pixel portion are formed over one substrate.

[0283] The thin film transistor to be disposed in the pixel portion is
formed according to any of Embodiments 1 to 4. Further, the thin film
transistor described in any of Embodiments 1 to 4 is an n-channel TFT.
Thus, part of a driver circuit that can be formed using n-channel TFTs
among driver circuits is formed over the same substrate as the thin film
transistor of the pixel portion.

[0284]FIG. 18A is an example of a block diagram of an active matrix
display device. Over a substrate 5300 in the display device, a pixel
portion 5301, a first scan line driver circuit 5302, a second scan line
driver circuit 5303, and a signal line driver circuit 5304 are provided.
In the pixel portion 5301, a plurality of signal lines which are extended
from the signal line driver circuit 5304 are provided and a plurality of
scan lines which are extended from the first scan line driver circuit
5302 and the second scan line driver circuit 5303 are provided. Note that
pixels which include display elements are provided in matrix in
respective regions where the scan lines and the signal lines intersect
with each other. Further, the substrate 5300 in the display device is
connected to a timing control circuit (also referred to as a controller
or a controller IC) through a connection portion such as a flexible
printed circuit (FPC).

[0285] In FIG. 18A, the first scan line driver circuit 5302, the second
scan line driver circuit 5303, and the signal line driver circuit 5304
are formed over the same substrate 5300 as the pixel portion 5301.
Accordingly, the number of components of a driver circuit and the like
provided outside is reduced, whereby reduction in cost can be achieved.
Further, if the driver circuit is provided outside the substrate 5300,
wirings would need to be extended and the number of connections of
wirings would be increased, but by providing the driver circuit over the
substrate 5300, the number of connections of the wirings can be reduced.
Accordingly, improvement in reliability and yield can be achieved.

[0286] Note that the timing control circuit 5305 supplies, for example, a
first scan line driver circuit start signal (GSP1) and a scan line driver
circuit clock signal (GCLK1) to the first scan line driver circuit 5302.
The timing control circuit 5305 supplies, for example, a second scan line
driver circuit start signal (GSP2) (also referred to as a start pulse)
and a scan line driver circuit clock signal (GCLK2) to the second scan
line driver circuit 5303. The timing control circuit 5305 supplies a
signal line driver circuit start signal (SSP), a signal line driver
circuit clock signal (SCLK), video signal data (DATA) (also simply
referred to as a video signal), and a latch signal (LAT) to the signal
line driver circuit 5304. Note that each clock signal may be a plurality
of clock signals whose periods are different or may be supplied together
with an inverted clock signal (CKB). Note that one of the first scan line
driver circuit 5302 and the second scan line driver circuit 5303 can be
omitted.

[0287]FIG. 18B illustrates a structure in which circuits with low driving
frequency (e.g., the first scan line driver circuit 5302 and the second
scan line driver circuit 5303) are formed over the same substrate 5300 as
the pixel portion 5301, and the signal line driver circuit 5304 is formed
over a substrate which is different from the substrate 5300 over which
the pixel portion 5301 is formed. With this structure, a driver circuit
formed over the substrate 5300 can be constituted by using thin film
transistors with lower field effect mobility as compared to that of a
transistor formed using a single crystal semiconductor. Accordingly,
increase in the size of the display device, reduction in the number of
steps, reduction in cost, improvement in yield, and the like can be
achieved.

[0288] The thin film transistor described in any of Embodiments 1 to 4 is
an n-channel TFT. In FIGS. 19A and 19B, an example of a structure and
operation of a signal line driver circuit which is formed using the
n-channel TFT is described as an example.

[0289] The signal line driver circuit includes a shift register 5601 and a
switching circuit 5602. The switching circuit 5602 includes a plurality
of switching circuits 5602_1 to 5602_N(N is a natural number). The
switching circuits 5602_1 to 5602_N each include a plurality of thin film
transistors 5603_1 to 5603--k (k is a natural number). An example in
which the thin film transistors 5603_1 to 5603--k are n-channel TFTs
will be described.

[0290] A connection relation of the signal line driver circuit will be
described by using the switching circuit 5602_1 as an example. First
terminals of the thin film transistors 5603_1 to 5603--k are
connected to wirings 5604_1 to 5604--k, respectively. Second
terminals of the thin film transistors 5603_1 to 5603--k are
connected to signal lines S1 to Sk, respectively. Gates of the thin film
transistors 5603_1 to 5603--k are connected to a wiring 5605_1.

[0291] The shift register 5601 has a function of sequentially selecting
the switching circuits 5602_1 to 5602_N by sequentially outputting
H-level signals (also referred to as H signals or signals at a high power
supply potential level) to wirings 5605_1 to 5605_N.

[0292] For example, the switching circuit 5602_1 has a function of
controlling conduction states between the wirings 5604_1 to 5604--k
and the signal lines S1 to Sk (conduction states between the first
terminals and the second terminals), that is, a function of controlling
whether or not to supply potentials of the wirings 5604_1 to 5604--k
to the signal lines S1 to Sk. In this manner, the switching circuit
5602_1 functions as a selector.

[0293] The thin film transistors 5603_1 to 5603--k have functions of
controlling conduction states between the wirings 5604_1 to 5604--k
and the signal lines S1 to Sk, that is, functions of supplying potentials
of the wirings 5604_1 to 5604--k to the signal lines S1 to Sk,
respectively. In this manner, each of the thin film transistors 5603_1 to
5603--k functions as a switch.

[0294] Note that video signal data (DATA) is input to each of the wirings
5604_1 to 5604--k. The video signal data (DATA) is an analog signal
corresponding to image data or image signals in many cases.

[0295] Next, operation of the signal line driver circuit in FIG. 19A is
described with reference to a timing chart in FIG. 19B. FIG. 19B
illustrates examples of signals Sout_1 to Sout_N and signals Vdata_1 to
Vdata_k. The signals Sout_1 to Sout_N are examples of output signals of
the shift register 5601, and the signals Vdata_1 to Vdata_k are examples
of signals which are input to the wirings 5604_1 to 5604--k. Note
that one operation period of the signal line driver circuit corresponds
to one gate selection period in a display device. For example, one gate
selection period is divided into periods T1 to TN. The periods T1 to TN
are periods for writing video signal data (DATA) to the pixels which
belong to a selected row.

[0296] Note that signal waveform distortion and the like in each structure
illustrated in drawings and the like in this embodiment are exaggerated
for simplicity in some cases. Therefore, this embodiment is not
necessarily limited to the scale illustrated in the drawings and the
like.

[0297] In the periods T1 to TN, the shift register 5601 sequentially
outputs H level signals to the wirings 5605_1 to 5605_N. For example, in
the period T1, the shift register 5601 outputs an H level signal to the
wiring 5605_1. Then, the thin film transistors 5603_1 to 5603--k are
turned on, so that the wirings 5604_1 to 5604--k and the signal
lines S1 to Sk are brought into conduction. In this case, Data (S1) to
Data (Sk) are input to the wirings 5604_1 to 5604--k, respectively.
The Data (S1) to Data (Sk) are input to pixels in a selected row in a
first to k-th columns through the thin film transistors 5603_1 to
5603--k, respectively. Thus, in the periods T1 to TN, video signal
data (DATA) is sequentially written to the pixels in the selected row of
every k columns.

[0298] By writing video signal data (DATA) to pixels of every plurality of
columns, the number of video signal data (DATA) or the number of wirings
can be reduced. Thus, connections to an external circuit can be reduced.
By writing video signals to pixels of every plurality of columns, writing
time can be extended and insufficient writing of video signals can be
prevented.

[0299] Note that as the shift register 5601 and the switching circuit
5602, a circuit including the thin film transistor described in
Embodiment 3 can be used. In this case, the shift register 5601 can be
constituted by only n-channel transistors or only p-channel transistors.

[0300] One embodiment of a shift register which is used for part of a scan
line driver circuit and/or a signal line driver circuit is described with
reference to FIGS. 20A to 20C and FIGS. 21A and 21B.

[0301] The scan line driver circuit includes a shift register. The scan
line driver circuit may also include a level shifter, a buffer, or the
like in some cases. In the scan line driver circuit, when a clock signal
(CK) and a start pulse signal (SP) are input to the shift register, a
selection signal is generated. The generated selection signal is buffered
and amplified by the buffer, and the resulting signal is supplied to a
corresponding scan line. Gate electrodes of transistors in pixels of one
line are connected to the scan line. Since the transistors in the pixels
of one line need to be turned on all at once, a buffer which can supply
large current is used.

[0302] The shift register includes first to N-th pulse output circuits
10_1 to 10_N(N is a natural number of 3 or more) (see FIG. 20A). A first
clock signal CK1 from a first wiring a second clock signal CK2 from a
second wiring 12, a third clock signal CK3 from a third wiring 13, and a
fourth clock signal CK4 from a fourth wiring 14 are supplied to the first
to N-th pulse output circuits 10_1 to 10_N of the shift register
illustrated in FIG. 20A. A start pulse SP1 (a first start pulse) from a
fifth wiring 15 is input to the first pulse output circuit 10_1. To the
n-th pulse output circuit 10--n of the second or subsequent stage (n
is a natural number greater than or equal to 2 and less than or equal to
N), a signal from the pulse output circuit of the preceding stage (such a
signal is referred to as a preceding-stage signal OUT(n-1)) (n is a
natural number greater than or equal to 2) is input. A signal from the
third pulse output circuit 10_3 which is two stages after the first pulse
output circuit 10_1 is input to the first pulse output circuit 10_1. In a
similar manner, to the n-th pulse output circuit 10--n of the second
or subsequent stage, a signal from the (n+2)-th pulse output circuit 10
(n+2) of the stage following the next stage (such a signal is referred to
as a subsequent-stage signal OUT(n+2)) is input. Therefore, the pulse
output circuits of the respective stages output first output signals
(OUT(1)(SR) to OUT(N)(SR)) to be input to the pulse output circuit of the
respective subsequent stage and/or the pulse output circuit of the stage
before the previous stage and second output signals (OUT(1) to OUT(N)) to
be electrically connected to another wiring or the like. Note that as
illustrated in FIG. 20A, the subsequent-stage signal OUT(n+2) is not
input to last two stages of the shift register; as an example, a second
start pulse SP2 and a third start pulse SP3 may be additionally input to
the last two stages of the shift register from a sixth wiring 16 and a
seventh wiring 17, respectively. Alternatively, a signal that is
additionally generated inside the shift register may be used. For
example, an (N+1)-th pulse output circuit 10 (N+1) and an (N+2)-th pulse
output circuit 10 (N+2) which do not contribute to pulse output to the
pixel portion (such circuits are also referred to as dummy stages) may be
provided so that signals corresponding to the second start pulse (SP2)
and the third start pulse (SP3) are generated in the dummy stages.

[0303] Note that a clock signal (CK) is a signal which alternates between
an H level and an L level (also referred to as an L signal or a signal at
a low power supply potential level) at regular intervals. The first to
the fourth clock signals (CK1) to (CK4) are delayed by 1/4 period
sequentially. In this embodiment, by using the first to fourth clock
signals (CK1) to (CK4), control of driving of a pulse output circuit or
the like is performed. Note that the clock signal is also called GCLK or
SCLK in accordance with a driver circuit to which the clock signal is
input; however, description is made using CK as the clock signal.

[0304] A first input terminal 21, a second input terminal 22, and a third
input terminal 23 are electrically connected to any of the first to
fourth wirings 11 to 14. For example, in FIG. 20A, the first input
terminal 21 of the first pulse output circuit 10_1 is electrically
connected to the first wiring 11, the second input terminal 22 of the
first pulse output circuit 10_1 is electrically connected to the second
wiring 12, and the third input terminal 23 of the first pulse output
circuit 10_1 is electrically connected to the third wiring 13. The first
input terminal 21 of the second pulse output circuit 10_2 is electrically
connected to the second wiring 12, the second input terminal 22 of the
second pulse output circuit 10_2 is electrically connected to the third
wiring 13, and the third input terminal 23 of the second pulse output
circuit 10_2 is electrically connected to the fourth wiring 14.

[0305] Each of the first to N-th pulse output circuits 10_1 to 10_N
includes the first input terminal 21, the second input terminal 22, the
third input terminal 23, the fourth input terminal 24, a fifth input
terminal 25, a first output terminal 26, and a second output terminal 27
(see FIG. 20B). In the first pulse output circuit 10_1, the first clock
signal CK1 is input to the first input terminal 21, the second clock
signal CK2 is input to the second input terminal 22, the third clock
signal CK3 is input to the third input terminal 23, the start pulse is
input to the fourth input terminal 24, the subsequent-stage signal OUT(3)
is input to the fifth input terminal 25, the first output signal
OUT(1)(SR) is output from the first output terminal 26, and the second
output signal OUT(1) is output from the second output terminal 27.

[0306] Next, an example of a specific circuit configuration of the pulse
output circuit is described with reference to FIG. 20C.

[0307] The first pulse output circuit 10_1 includes first to eleventh
transistors 31 to 41 (see FIG. 20C). Signals or power supply potentials
are supplied to the first to eleventh transistors 31 to 41 from a power
supply line 51 to which a first high power supply potential VDD is
supplied, a power supply line 52 to which a second high power supply
potential VCC is supplied, and a power supply line 53 to which a low
power supply potential VSS is supplied, in addition to the above first to
fifth input terminals 21 to 25 and the first and second output terminals
26 and 27. Here, a magnitude relation of the power supply potentials of
the power supply lines in FIG. 20C is as follows: the first power supply
potential VDD is higher than or equal to the second power supply
potential VCC, and the second power supply potential VCC is higher than
the third power supply potential VSS. Note that the first to fourth clock
signals (CK1) to (CK4) are each a signal which alternates between an H
level and an L level at regular intervals; the clock signal at the H
level is VDD, and the clock signal at the L level is VSS. Note that when
the potential VDD of the power supply line 51 is set to be higher than
the potential VCC of the power supply line 52, a potential applied to the
gate electrode of the transistor can be reduced without affecting the
operation; thus, the shift of the threshold value of the transistor can
be reduced and deterioration can be suppressed.

[0308] In FIG. 20C, a first terminal of the first transistor 31 is
electrically connected to the power supply line 51, a second terminal of
the first transistor 31 is electrically connected to a first terminal of
the ninth transistor 39, and a gate electrode of the first transistor 31
is electrically connected to the fourth input terminal 24. A first
terminal of the second transistor 32 is electrically connected to the
power supply line 53, a second terminal of the second transistor 32 is
electrically connected to the first terminal of the ninth transistor 39,
and a gate electrode of the second transistor 32 is electrically
connected to a gate electrode of the fourth transistor 34. A first
terminal of the third transistor 33 is electrically connected to the
first input terminal 21, and a second terminal of the third transistor 33
is electrically connected to the first output terminal 26. A first
terminal of the fourth transistor 34 is electrically connected to the
power supply line 53, and a second terminal of the fourth transistor 34
is electrically connected to the first output terminal 26. A first
terminal of the fifth transistor 35 is electrically connected to the
power supply line 53, a second terminal of the fifth transistor 35 is
electrically connected to the gate electrode of the second transistor 32
and the gate electrode of the fourth transistor 34, and a gate electrode
of the fifth transistor 35 is electrically connected to the fourth input
terminal 24. A first terminal of the sixth transistor 36 is electrically
connected to the power supply line 52, a second terminal of the sixth
transistor 36 is electrically connected to the gate electrode of the
second transistor 32 and the gate electrode of the fourth transistor 34,
and a gate electrode of the sixth transistor 36 is electrically connected
to the fifth input terminal 25. A first terminal of the seventh
transistor 37 is electrically connected to the power supply line 52, a
second terminal of the seventh transistor 37 is electrically connected to
a second terminal of the eighth transistor 38, and a gate electrode of
the seventh transistor 37 is electrically connected to the third input
terminal 23. A first terminal of the eighth transistor 38 is electrically
connected to the gate electrode of the second transistor 32 and the gate
electrode of the fourth transistor 34, and a gate electrode of the eighth
transistor 38 is electrically connected to the second input terminal 22.
The first terminal of the ninth transistor 39 is electrically connected
to the second terminal of the first transistor 31 and the second terminal
of the second transistor 32, a second terminal of the ninth transistor 39
is electrically connected to a gate electrode of the third transistor 33
and a gate electrode of the tenth transistor 40, and a gate electrode of
the ninth transistor 39 is electrically connected to the power supply
line 52. A first terminal of the tenth transistor 40 is electrically
connected to the first input terminal 21, a second terminal of the tenth
transistor 40 is electrically connected to the second output terminal 27,
and the gate electrode of the tenth transistor 40 is electrically
connected to the second terminal of the ninth transistor 39. A first
terminal of the eleventh transistor 41 is electrically connected to the
power supply line 53, a second terminal of the eleventh transistor 41 is
electrically connected to the second output terminal 27, and a gate
electrode of the eleventh transistor 41 is electrically connected to the
gate electrode of the second transistor 32 and the gate electrode of the
fourth transistor 34.

[0309] In FIG. 20C, a portion where the gate electrode of the third
transistor 33, the gate electrode of the tenth transistor 40, and the
second terminal of the ninth transistor 39 are connected is referred to
as a node A. In addition, a portion where the gate electrode of the
second transistor 32, the gate electrode of the fourth transistor 34, the
second terminal of the fifth transistor 35, the second terminal of the
sixth transistor 36, the first terminal of the eighth transistor 38, and
the gate electrode of the eleventh transistor 41 is referred to as a node
B (see FIG. 21A).

[0310] In FIG. 21A, signals which are input to or output from the first to
the fifth input terminals 21 to 25, the first output terminal 26, and the
second output terminal 27 when the pulse output circuit illustrated in
FIG. 20C is applied to the first pulse output circuit 10_1 are
illustrated.

[0311] Specifically, the first clock signal CK1 is input to the first
input terminal 21, the second clock signal CK2 is input to the second
input terminal 22, the third clock signal CK3 is input to the third input
terminal 23, the start pulse is input to the fourth input terminal 24,
the subsequent-stage signal OUT (3) is input to the fifth input terminal
25, the first output signal OUT(1)(SR) is output from the first output
terminal 26, and the second output signal OUT(1) is output from the
second output terminal 27.

[0312] Note that a thin film transistor is an element having at least
three terminals of a gate, a drain, and a source. The thin film
transistor has a semiconductor in which a channel region is formed in a
region overlapping with the gate, and current which flows between the
drain and the source through the channel region can be controlled by
controlling the potential of the gate. Here, since the source and the
drain of the thin film transistor may interchange depending on the
structure, the operating conditions, or the like of the thin film
transistor, it is difficult to determine which is the source and which is
the drain. Therefore, a region functioning as a source or a drain is not
called the source or the drain in some cases. In that case, for example,
such regions may be referred to as a first terminal and a second
terminal.

[0313] Here, FIG. 21B is a timing chart of the shift register including
the plurality of pulse output circuits illustrated in FIG. 21A. Note that
when the shift register is included in a scan line driver circuit, a
period 61 and a period 62 in FIG. 21B correspond to a vertical retrace
period and a gate selection period, respectively.

[0314] Note that as illustrated in FIG. 21A, by providing the ninth
transistor 39 whose gate is supplied with the second power supply
potential VCC, advantages described below are obtained before and after a
bootstrap operation.

[0315] Without the ninth transistor 39 whose gate is supplied with the
second power supply potential VCC, if the potential of the node A is
raised by the bootstrap operation, the potential of a source which is the
second terminal of the first transistor 31 increases to a value higher
than the first power supply potential VDD. Then, the source of the first
transistor 31 is switched to the first terminal, that is, the terminal on
the power supply line 51 side. Therefore, in the first transistor 31,
large bias voltage is applied and thus significant stress is applied
between the gate and the source and between the gate and the drain, which
can cause deterioration of the transistor. By providing the ninth
transistor 39 whose gate is supplied the second power supply potential
VCC, the potential of the node A is raised by the bootstrap operation,
but increase in the potential of the second terminal of the first
transistor 31 can be prevented. In other words, by providing the ninth
transistor 39, negative bias voltage applied between the gate and the
source of the first transistor 31 can be reduced. Accordingly, with the
circuit configuration of this embodiment, negative bias voltage applied
between the gate and the source of the first transistor 31 can be
reduced, so that deterioration in the first transistor 31 due to stress
can further be suppressed.

[0316] Note that the ninth transistor 39 is provided so as to be connected
between the second terminal of the first transistor 31 and the gate of
the third transistor 33 through the first terminal and the second
terminal thereof. When the shift register including a plurality of the
pulse output circuits described in this embodiment is used, in a signal
line driver circuit having more stages than a scan line driver circuit,
the ninth transistor 39 may be omitted, which is advantageous in that the
number of transistors can be reduced.

[0317] When an oxide semiconductor is used for semiconductor layers of the
first to eleventh transistor 31 to 41, off current of the thin film
transistor can be reduced, on current and field effect mobility can be
increased, and the degree of deterioration can be decreased; thus a
malfunction in a circuit can be reduced. The degree of deterioration of
the transistor formed using an oxide semiconductor, which is caused by
application of a high potential to the gate electrode, is small as
compared to that of a transistor formed using amorphous silicon.
Therefore, even when the first power supply potential VDD is supplied to
a power supply line to which the second power supply potential VCC is
supplied, similar operation can be performed, and the number of power
supply lines which are provided in a circuit can be reduced, so that the
circuit can be miniaturized.

[0318] Note that even if a wiring connection is changed so that the clock
signal which is supplied to the gate electrode of the seventh transistor
37 from the third input terminal 23 and the clock signal which is
supplied to the gate electrode of the eighth transistor 38 from the
second input terminal 22 are a clock signal which is supplied to the gate
electrode of the seventh transistor 37 from the second input terminal 22
and a clock signal which is supplied to the gate electrode of the eighth
transistor 38 from the third input terminal 23, respectively, a similar
effect can be obtained. At this time, in the shift register illustrated
in FIG. 21A, after the seventh transistor 37 and the eighth transistor 38
are both turned on, the seventh transistor 37 is turned off and the
eighth transistor 38 is still on, and then the seventh transistor 37 is
still off and the eighth transistor 38 is turned off. Thus, fall in the
potential of the node B, which is caused by fall in the potentials of the
second input terminal 22 and the third input terminal 23, occurs twice
because of fall in the potential of the gate electrode of the seventh
transistor 37 and fall in the potential of the gate electrode of the
eighth transistor 38. On the other hand, when states of the seventh
transistor 37 and the eighth transistor 38 in the shift register
illustrated in FIG. 21A is changed so that both the seventh transistor 37
and the eighth transistor 38 are on, then the seventh transistor 37 is on
and the eighth transistor 38 is off, and then the seventh transistor 37
and the eighth transistor 38 are off, the number of falls in the
potential of the node B, which is caused by fall in the potentials of the
second input terminal 22 and the third input terminal 23, can be reduced
to one time, which is caused by fall in the potential of the gate
electrode of the eighth transistor 38. Therefore, the connection relation
in which the clock signal CK3 is supplied from the third input terminal
23 to the gate electrode of the seventh transistor 37 and the clock
signal CK2 is supplied from the second input terminal 22 to the gate
electrode of the eighth transistor 38 is preferable. That is because the
number of times of the change in the potential of the node B can be
reduced, whereby the noise can be reduced.

[0319] In this manner, in a period during which the potentials of the
first output terminal 26 and the second output terminal 27 are held at
the L level, the H level signal is regularly supplied to the node B;
accordingly, a malfunction of the pulse output circuit can be suppressed.

[0320] This embodiment can be combined with any of the structures
described in the other embodiments as appropriate.

Embodiment 7

[0321] When a thin film transistor is manufactured and used for a pixel
portion and further for a driver circuit, a semiconductor device having a
display function (also referred to as a display device) can be
manufactured. Furthermore, when part or whole of a driver circuit is
formed over the same substrate as a pixel portion, a system-on-panel can
be obtained.

[0322] The display device includes a display element. As the display
element, a liquid crystal element (also referred to as a liquid crystal
display element) or a light-emitting element (also referred to as a
light-emitting display element) can be used. Light-emitting elements
include, in its category, an element whose luminance is controlled by
current or voltage, and specifically include an inorganic
electroluminescent (EL) element, an organic EL element, and the like.
Furthermore, a display medium whose contrast is changed by an electric
action, such as electronic ink, can be used.

[0323] The display device includes a panel in which the display element is
sealed, and a module in which an IC or the like including a controller is
mounted on the panel. The display device relates to one embodiment of an
element substrate before the display element is completed in a
manufacturing process of the display device, and the element substrate is
provided with a means for supplying current to the display element in
each of a plurality of pixels. Specifically, the element substrate may be
in a state after only a pixel electrode (also referred to as a pixel
electrode layer) of the display element is formed, a state after a
conductive film to be a pixel electrode is formed and before the
conductive film is etched to form the pixel electrode, or any of other
states.

[0324] Note that a display device in this specification means an image
display device, a display device, or a light source (including a lighting
device). Furthermore, the display device also includes the following
modules in its category: a module to which a connector such as a flexible
printed circuit (FPC), a tape automated bonding (TAB) tape, or a tape
carrier package (TCP) is attached; a module having a TAB tape or a TCP at
the tip of which a printed wiring board is provided; and a module in
which an integrated circuit (IC) is directly mounted on a display element
by a chip on glass (COG) method.

[0325] The appearance and a cross section of a liquid crystal display
panel, which is one embodiment of a semiconductor device, will be
described with reference to FIGS. 14A1, 14A2 and 14B. FIGS. 14A1 and 14A2
are each a plan view of a panel in which thin film transistors 4010 and
4011 and a liquid crystal element 4013 are sealed between a first
substrate 4001 and a second substrate 4006 with a sealant 4005. FIG. 14B
is a cross-sectional view taken along line M-N of FIGS. 14A1 and 14A2.

[0326] The sealant 4005 is provided so as to surround a pixel portion 4002
and a scan line driver circuit 4004 that are provided over the first
substrate 4001. The second substrate 4006 is provided over the pixel
portion 4002 and the scan line driver circuit 4004. Therefore, the pixel
portion 4002 and the scan line driver circuit 4004 are sealed together
with a liquid crystal layer 4008, by the first substrate 4001, the
sealant 4005, and the second substrate 4006. A signal line driver circuit
4003 that is formed using a single crystal semiconductor film or a
polycrystalline semiconductor film over a substrate separately prepared
is mounted in a region different from the region surrounded by the
sealant 4005 over the first substrate 4001.

[0327] Note that there is no particular limitation on the connection
method of the driver circuit which is separately formed, and a COG
method, a wire bonding method, a TAB method, or the like can be used.
FIG. 14A1 illustrates an example of mounting the signal line driver
circuit 4003 by a COG method, and FIG. 14A2 illustrates an example of
mounting the signal line driver circuit 4003 by a TAB method.

[0328] The pixel portion 4002 and the scan line driver circuit 4004
provided over the first substrate 4001 each include a plurality of thin
film transistors. FIG. 14B illustrates the thin film transistor 4010
included in the pixel portion 4002 and the thin film transistor 4011
included in the scan line driver circuit 4004 as an example. Insulating
layers 4041, 4020, 4042, and 4021 are provided over the thin film
transistors 4010 and 4011. Further, an insulating layer 4043 is provided
over the first substrate 4001, and an insulating layer 4044 and an
insulating layer 4045 are provided over the gate electrode layers of the
thin film transistors. A source wiring 4046 is provided over the
insulating layer 4020 and connected to a source electrode or a drain
electrode of the thin film transistor 4010 through a contact hole formed
in the insulating layer 4020 and the insulating layer 4041.

[0329] As the thin film transistors 4010 and 4011, the highly-reliable
thin film transistor including an oxide semiconductor layer, which is
described in any of Embodiments 1 to 4, can be employed. In this
embodiment, the thin film transistors 4010 and 4011 are re-channel thin
film transistors.

[0330] A conductive layer 4040 is provided over the insulating layer 4021,
which overlaps with a channel formation region of an oxide semiconductor
layer in the thin film transistor 4011 for the driver circuit. The
conductive layer 4040 is provided in the position overlapping with the
channel formation region of the oxide semiconductor layer, whereby the
amount of shift in the threshold voltage of the thin film transistor 4011
before and after a BT test can be reduced. The potential of the
conductive layer 4040 may be the same as or different from that of a gate
electrode layer of the thin film transistor 4011. The conductive layer
4040 can also function as a second gate electrode layer. Alternatively,
the potential of the conductive layer 4040 may be GND or 0 V, or the
conductive layer 4040 may be in a floating state.

[0331] Note that a thin film transistor manufactured in accordance with
the process described in Embodiment 2 includes a highly purified oxide
semiconductor layer. Specifically, in order to prevent entry of an
impurity (e.g., a hydrogen atom, a compound including a hydrogen atom
such as H2O, or a compound including a carbon atom), evacuation with
a cryopump or the like is performed at the time of forming the oxide
semiconductor layer. Further, the oxide semiconductor layer is subjected
to heat treatment for dehydration or dehydrogenation after its formation.
Furthermore, an oxide insulating film is formed in a region where a
so-called back channel of the thin film transistor is formed, whereby
impurities move from the oxide semiconductor layer into the oxide
insulating film.

[0332] In addition, by providing the conductive layer 4040 in a position
overlapping with the channel formation region, the thin film transistor
is shielded from static electricity. When the thin film transistor is
shielded from static electricity, the number of carriers due to static
electricity can be reduced.

[0333] When the oxide semiconductor layer is highly purified and shielded
from static electricity, the carrier density of the oxide semiconductor
layer is reduced. For example, the carrier density of the oxide
semiconductor layer can be suppressed to 1×1014/cm3 or
lower. In this manner, by using an oxide semiconductor layer with a
suppressed carrier density for a thin film transistor, the transistor can
have small off current (Ioff). Moreover, by applying the thin film
transistor whose off current (Ioff) is suppressed to a display
device, the display device can have low power consumption.

[0334] A pixel electrode layer 4030 included in the liquid crystal element
4013 is electrically connected to the thin film transistor 4010. A
counter electrode layer 4031 of the liquid crystal element 4013 is formed
on the second substrate 4006. A portion where the pixel electrode layer
4030, the counter electrode layer 4031, and the liquid crystal layer 4008
overlap with one another corresponds to the liquid crystal element 4013.
Note that the pixel electrode layer 4030 and the counter electrode layer
4031 are provided with an insulating layer 4032 and an insulating layer
4033, respectively, each of which functions as an alignment film. The
liquid crystal layer 4008 is sandwiched between the pixel electrode layer
4030 and the counter electrode layer 4031 with the insulating layers 4032
and 4033 interposed therebetween.

[0335] Note that as the first substrate 4001 and the second substrate
4006, a light-transmitting substrate can be employed, and glass, ceramic,
or plastic can be used. As plastic, a fiberglass-reinforced plastics
(FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film, or an
acrylic resin film can be used.

[0336] A spacer 4035 is a columnar spacer obtained by selective etching of
an insulating film and is provided in order to control the distance (a
cell gap) between the pixel electrode layer 4030 and the counter
electrode layer 4031. Note that a spherical spacer may be used as the
spacer 4035. The counter electrode layer 4031 is electrically connected
to a common potential line provided over the same substrate as the thin
film transistor 4010. With the use of the common connection portion, the
counter electrode layer 4031 can be electrically connected to the common
potential line through conductive particles provided between the pair of
substrates. Note that the conductive particles are included in the
sealant 4005.

[0337] Alternatively, liquid crystal exhibiting a blue phase for which an
alignment film is unnecessary may be used. A blue phase is one of the
liquid crystal phases, which is generated just before a cholesteric phase
changes into an isotropic phase while temperature of cholesteric liquid
crystal is raised. Since the blue phase is generated within a narrow
range of temperature, a liquid crystal composition including a chiral
agent at 5 weight % or more is used for the liquid crystal layer 4008 in
order to improve the temperature range. The liquid crystal composition
which includes liquid crystal exhibiting a blue phase and a chiral agent
has short response time of 1 msec or less, has optical isotropy, which
makes the alignment treatment unneeded, and has a small viewing angle
dependence.

[0338] An embodiment of the present invention can also be applied to a
transflective liquid crystal display device, in addition to a
transmissive liquid crystal display device.

[0339] An example of the liquid crystal display device is described in
which a polarizing plate is provided on the outer surface of the
substrate (on the viewer side) and a coloring layer (color filter) and an
electrode layer used for a display element are provided on the inner
surface of the substrate in this order; however, the polarizing plate may
be provided on the inner surface of the substrate. The stacked-layer
structure of the polarizing plate and the coloring layer is not limited
to that of this embodiment and may be set as appropriate in accordance
with materials of the polarizing plate and the coloring layer or
conditions of the manufacturing process.

[0340] Over the thin film transistors 4010 and 4011, the insulating layer
4041 is formed as a protective insulating film to be in contact with the
semiconductor layer including the channel formation region. The
insulating layer 4041 may be formed using a material and a method similar
to those of the insulating layer 208 described in Embodiments 1 and 2,
for example. Here, a silicon oxide film is formed as the insulating layer
4041 by a sputtering method in a manner similar to that of Embodiments 1
and 2.

[0341] In order to reduce surface roughness of the thin film transistors,
the insulating layer 4021 functioning as a planarization insulating film
is formed over the insulating layer 4020. As the insulating layer 4021,
an organic material having heat resistance such as polyimide, acrylic,
benzocyclobutene, polyamide, or epoxy can be used. Other than such
organic materials, it is also possible to use a low-dielectric constant
material (a low-k material), a siloxane-based resin, phosphosilicate
glass (PSG), borophosphosilicate glass (BPSG), or the like. Note that the
insulating layer 4021 may be formed by stacking a plurality of insulating
films formed using any of these materials.

[0342] Note that a siloxane-based resin is a resin which is formed from a
siloxane-based material as a starting material and has a Si--O--Si bond.
The siloxane-based resin may include as a substituent an organic group
(e.g., an alkyl group or an aryl group) or a fluoro group. The organic
group may include a fluoro group.

[0343] There is no particular limitation on the method for forming the
insulating layer 4021, and the insulating layer 4021 can be formed,
depending on the material, by a sputtering method, an SOG method, spin
coating, dipping, spray coating, a droplet discharge method (such as an
inkjet method, screen printing, or offset printing), or with a tool such
as a doctor knife, a roll coater, a curtain coater, or a knife coater.
When a baking step of the insulating layer 4021 also serves as the
annealing step of the semiconductor layer, the semiconductor device can
be manufactured efficiently.

[0344] The pixel electrode layer 4030 and the counter electrode layer 4031
can be formed using a light-transmitting conductive material such as
indium oxide including tungsten oxide, indium zinc oxide including
tungsten oxide, indium oxide including titanium oxide, indium tin oxide
including titanium oxide, indium tin oxide (hereinafter referred to as
ITO), indium zinc oxide, or indium tin oxide to which silicon oxide is
added.

[0345] A conductive composition including a conductive high molecule (also
referred to as a conductive polymer) can be used for the pixel electrode
layer 4030 and the counter electrode layer 4031. A pixel electrode formed
using the conductive composition preferably has a sheet resistance of
10000 ohms per square or less and a light transmittance of 70% or higher
at a wavelength of 550 nm. Furthermore, the resistivity of the conductive
high molecule included in the conductive composition is preferably 0.1
Ωcm or lower.

[0346] As the conductive high molecule, a so-called π-electron
conjugated conductive polymer can be used. For example, it is possible to
use polyaniline or a derivative thereof, polypyrrole or a derivative
thereof, polythiophene or a derivative thereof, or a copolymer of two or
more kinds of them.

[0347] A variety of signals and potentials are supplied from an FPC 4018
to the signal line driver circuit 4003 that is formed separately, and the
scan line driver circuit 4004 or the pixel portion 4002.

[0348] A connection terminal electrode 4015 is formed from the same
conductive film as the pixel electrode layer 4030 included in the liquid
crystal element 4013, and a terminal electrode 4016 is formed from the
same conductive film as source electrode layers and drain electrode
layers of the thin film transistors 4010 and 4011.

[0349] The connection terminal electrode 4015 is electrically connected to
a terminal included in the FPC 4018 through an anisotropic conductive
film 4019.

[0350] FIGS. 14A1, 14A2 and 14B illustrate an example in which the signal
line driver circuit 4003 is formed separately and mounted on the first
substrate 4001; however, this embodiment is not limited to this
structure. The scan line driver circuit may be separately formed and then
mounted, or only part of the signal line driver circuit or part of the
scan line driver circuit may be separately formed and then mounted.

[0351]FIG. 23 illustrates an example of a liquid crystal display module
which is formed as a semiconductor device with the use of a TFT substrate
2600 manufactured in accordance with the manufacturing method disclosed
in this specification.

[0352]FIG. 23 illustrates an example of a liquid crystal display module,
in which the TFT substrate 2600 and a counter substrate 2601 are bonded
to each other with a sealant 2602, and a pixel portion 2603 including a
TFT or the like, a display element 2604 including a liquid crystal layer,
and a coloring layer 2605 are provided between the substrates to form a
display region. The coloring layer 2605 is necessary to perform color
display. In the case of the RGB system, respective coloring layers
corresponding to colors of red, green, and blue are provided for
respective pixels. Polarizing plates 2606 and 2607 and a diffusion plate
2613 are provided outside the TFT substrate 2600 and the counter
substrate 2601. A light source includes a cold cathode tube 2610 and a
reflection plate 2611. A circuit board 2612 is connected to a wiring
circuit portion 2608 of the TFT substrate 2600 through a flexible wiring
board 2609 and includes an external circuit such as a control circuit or
a power supply circuit. The polarizing plate and the liquid crystal layer
may be stacked with a retardation plate interposed therebetween.

[0354] Through the above process, a highly reliable liquid crystal display
panel as a semiconductor device can be manufactured.

[0355] By manufacturing the above liquid crystal display device with the
use of the display device described in any of Embodiments 1 to 5, a gate
wiring or a source wiring can be formed using a conductive material
including Cu; accordingly, increase in wiring resistance can be
prevented. Consequently, high speed operation and low power consumption
of the above liquid crystal display device can be achieved, and thus the
liquid crystal display device can have a large-sized screen or a high
definition screen.

[0356] This embodiment can be combined with any of the structures
described in the other embodiments as appropriate.

Embodiment 8

[0357] In this embodiment, an example of electronic paper will be
described as one embodiment of a semiconductor device.

[0358] The thin film transistor of Embodiment 1 may be used for electronic
paper in which electronic ink is driven by an element electrically
connected to a switching element. The electronic paper is also referred
to as an electrophoretic display device (an electrophoretic display) and
is advantageous in that it has the same level of readability as plain
paper, it has lower power consumption than other display devices, and it
can be made thin and lightweight.

[0359] Electrophoretic displays can have various modes. An electrophoretic
display includes a plurality of microcapsules dispersed in a solvent or a
solute, and each microcapsule includes first particles which are
positively charged and second particles which are negatively charged. By
applying an electric field to the microcapsules, the particles in the
microcapsules move in opposite directions and only the color of the
particles gathering on one side is displayed. Note that the first
particles and the second particles each include a pigment and do not move
without an electric field. In addition, the first particles and the
second particles have different colors (which may be colorless).

[0360] Thus, an electrophoretic display is a display that utilizes a
so-called dielectrophoretic effect by which a substance having a high
dielectric constant moves to a high electric field region.

[0361] A solution in which the above microcapsules are dispersed in a
solvent is called electronic ink. This electronic ink can be printed on a
surface of glass, plastic, cloth, paper, or the like. Furthermore, by
using a color filter or particles that have a pigment, color display can
also be achieved.

[0362] In addition, if a plurality of the above microcapsules is arranged
as appropriate over an active matrix substrate so as to be interposed
between two electrodes, an active matrix display device can be completed
and display can be performed by application of an electric field to the
microcapsules. For example, the active matrix substrate obtained using
the thin film transistor described in Embodiment 1 can be used.

[0363] Note that the first particles and the second particles in the
microcapsules may be formed using a single material selected from a
conductive material, an insulating material, a semiconductor material, a
magnetic material, a liquid crystal material, a ferroelectric material,
an electroluminescent material, an electrochromic material, and a
magnetophoretic material, or formed using a composite material of any of
these materials.

[0364]FIG. 22 illustrates active matrix electronic paper as an example of
a semiconductor device. A thin film transistor 581 used for the
semiconductor device can be manufactured in a manner similar to that of
the thin film transistors described in Embodiments 1 and 2 and is a
highly reliable thin film transistor including an oxide semiconductor
layer.

[0365] The electronic paper in FIG. 22 is an example of a display device
using a twisting ball display system. The twisting ball display system
refers to a method in which spherical particles each colored in black and
white are arranged between a first electrode layer and a second electrode
layer which are electrode layers used for a display element, and a
potential difference is generated between the first electrode layer and
the second electrode layer to control orientation of the spherical
particles, so that display is performed.

[0366] The thin film transistor 581 formed over a substrate 580 is a thin
film transistor having a bottom gate structure and is covered with an
insulating layer 583 which is in contact with the semiconductor layer. An
insulating layer 591 is formed over the substrate 580, an insulating
layer 592 and an insulating layer 582 are formed over the gate electrode
layer of the thin film transistor, and an insulating layer 597 and an
insulating layer 598 are formed over the insulating layer 583. Further, a
source wiring 599a and a source wiring 599b are formed over the
insulating layer 583 and connected to a source electrode or a drain
electrode of the thin film transistor 581 through a contact hole formed
in the insulating layer 583 and the insulating layer 597. The source
electrode layer or the drain electrode layer of the thin film transistor
581 is in contact with a first electrode layer 587 through an opening
formed in an insulating layer 585, whereby the thin film transistor 581
is electrically connected to the first electrode layer 587. Spherical
particles 589 are provided between the first electrode layer 587 and a
second electrode layer 588 formed on a substrate 596. Each of the
spherical particles 589 includes a black region 590a, a white region
590b, and a cavity 594 filled with liquid around the black region 590a
and the white region 590b. A space around the spherical particles 589 is
filled with a filler 595 such as a resin (see FIG. 22). The first
electrode layer 587 corresponds to a pixel electrode, and the second
electrode layer 588 corresponds to a common electrode. The second
electrode layer 588 is electrically connected to a common potential line
provided over the same substrate as the thin film transistor 581. With
the use of a common connection portion, the second electrode layer 588
can be electrically connected to the common potential line through
conductive particles provided between the pair of substrates.

[0367] Instead of the twisting ball, an electrophoretic element can be
used. A microcapsule with a diameter of approximately 10 μm to 200
μm in which transparent liquid, positively-charged white
microparticles, and negatively-charged black microparticles are
encapsulated, is used. In the microcapsule which is provided between the
first electrode layer and the second electrode layer, when an electric
field is applied between the first electrode layer and the second
electrode layer, the white microparticles and the black microparticles
move to opposite sides from each other, so that white or black can be
displayed. A display element using this principle is an electrophoretic
display element and is generally called electronic paper. The
electrophoretic display element has higher reflectance than a liquid
crystal display element, and thus, an auxiliary light is unnecessary,
power consumption is low, and a display portion can be recognized even in
a dim environment. Moreover, even when power is not supplied to the
display portion, an image which has been displayed once can be
maintained. Accordingly, a displayed image can be stored even if a
semiconductor device having a display function (which may be referred to
simply as a display device or a semiconductor device provided with a
display device) is apart from an electric wave source.

[0368] Through the above process, highly reliable electronic paper as a
semiconductor device can be manufactured.

[0369] In the case where a thin film transistor in a pixel portion of the
above electronic paper is manufactured using any of methods for
manufacturing the thin film transistors described in Embodiments 1 to 3,
display unevenness due to variation in the threshold voltage of thin film
transistors of respective pixels can be suppressed.

[0370] By manufacturing the above electronic paper with the use of the
display device described in any of Embodiments 1 to 5, a gate wiring or a
source wiring can be formed using a conductive material including Cu;
accordingly, increase in wiring resistance can be prevented.
Consequently, high speed operation and low power consumption of the above
electronic paper can be achieved, and thus the electronic paper can have
a large-sized screen or a high definition screen.

[0371] This embodiment can be combined with any of the structures
described in the other embodiments as appropriate.

Embodiment 9

[0372] An example of a light-emitting display device will be described as
a semiconductor device. As a display element included in the display
device, a light-emitting element utilizing electroluminescence is
described here. Light-emitting elements utilizing electroluminescence are
classified according to whether a light-emitting material is an organic
compound or an inorganic compound. In general, the former is referred to
as an organic EL element and the latter is referred to as an inorganic EL
element.

[0373] In an organic EL element, by application of voltage to a
light-emitting element, electrons and holes are separately injected from
a pair of electrodes into a layer including a light-emitting organic
compound, and current flows. Then, the carriers (electrons and holes)
recombine, so that the light-emitting organic compound is excited. The
light-emitting organic compound returns to a ground state from the
excited state, thereby emitting light. Owing to such a mechanism, this
light-emitting element is referred to as a current-excitation
light-emitting element.

[0374] The inorganic EL elements are classified according to their element
structures into a dispersion-type inorganic EL element and a thin-film
inorganic EL element. A dispersion-type inorganic EL element has a
light-emitting layer where particles of a light-emitting material are
dispersed in a binder, and its light emission mechanism is donor-acceptor
recombination type light emission which utilizes a donor level and an
acceptor level. A thin film inorganic EL element has a structure where a
light-emitting layer is sandwiched between dielectric layers, which are
further sandwiched between electrodes, and its light emission mechanism
is localized type light emission that utilizes inner-shell electron
transition of metal ions. Note that description is made here using an
organic EL element as a light-emitting element.

[0375]FIG. 16 illustrates an example of a pixel configuration as an
example of a semiconductor device, which can be driven by a digital time
grayscale method.

[0376] The configuration and operation of a pixel which can be driven by a
digital time grayscale method will be described. An example is described
here in which one pixel includes two n-channel transistors using an oxide
semiconductor layer in a channel formation region.

[0377] A pixel 6400 includes a switching transistor 6401, a light-emitting
element driving transistor 6402, a light-emitting element 6404, and a
capacitor 6403. A gate of the switching transistor 6401 is connected to a
scan line 6406, a first electrode (one of a source electrode and a drain
electrode) of the switching transistor 6401 is connected to a signal line
6405, and a second electrode (the other of the source electrode and the
drain electrode) of the switching transistor 6401 is connected to a gate
of the light-emitting element driving transistor 6402. The gate of the
light-emitting element driving transistor 6402 is connected to a power
supply line 6407 through the capacitor 6403, a first electrode of the
light-emitting element driving transistor 6402 is connected to the power
supply line 6407, and a second electrode of the light-emitting element
driving transistor 6402 is connected to a first electrode (a pixel
electrode) of the light-emitting element 6404. A second electrode of the
light-emitting element 6404 corresponds to a common electrode 6408. The
common electrode 6408 is electrically connected to a common potential
line formed over the same substrate.

[0378] Note that the second electrode (the common electrode 6408) of the
light-emitting element 6404 is set to a low power supply potential. The
low power supply potential is lower than a high power supply potential
which is supplied to the power supply line 6407 when the high power
supply potential is a reference. For example, GND and 0 V may be set as
the low power supply potential. The potential difference between the high
power supply potential and the low power supply potential is applied to
the light-emitting element 6404 so that current flows through the
light-emitting element 6404, whereby the light-emitting element 6404
emits light. Thus, each potential is set so that the potential difference
between the high power supply potential and the low power supply
potential is greater than or equal to forward threshold voltage of the
light-emitting element 6404.

[0379] When the gate capacitance of the light-emitting element driving
transistor 6402 is used as a substitute for the capacitor 6403, the
capacitor 6403 can be omitted. The gate capacitance of the light-emitting
element driving transistor 6402 may be formed between a channel region
and a gate electrode.

[0380] Here, in the case of using a voltage-input voltage driving method,
a video signal is input to the gate of the light-emitting element driving
transistor 6402 so that the light-emitting element driving transistor
6402 is sufficiently turned on or off. That is, the light-emitting
element driving transistor 6402 operates in a linear region, and thus
voltage higher than the voltage of the power supply line 6407 is applied
to the gate electrode of the light-emitting element driving transistor
6402. Note that voltage higher than or equal to (power supply line
voltage+Vth of the light-emitting element driving transistor 6402)
is applied to the signal line 6405.

[0381] In the case of using an analog grayscale method instead of the
digital time grayscale method, the same pixel configuration as in FIG. 16
can be employed by inputting signals in a different way.

[0382] In the case of using the analog grayscale method, voltage higher
than or equal to (forward voltage of the light-emitting element
6404+Vth of the light-emitting element driving transistor 6402) is
applied to the gate electrode of the light-emitting element driving
transistor 6402. The forward voltage of the light-emitting element 6404
refers to voltage at which a desired luminance is obtained, and includes
at least forward threshold voltage. By inputting a video signal to enable
the light-emitting element driving transistor 6402 to operate in a
saturation region, current can be supplied to the light-emitting element
6404. In order that the light-emitting element driving transistor 6402
can operate in the saturation region, the potential of the power supply
line 6407 is set higher than a gate potential of the light-emitting
element driving transistor 6402. Since the video signal is an analog
signal, current according to the video signal flows in the light-emitting
element 6404, and analog grayscale driving can be performed.

[0383] Note that the pixel configuration is not limited to that
illustrated in FIG. 16. For example, the pixel illustrated in FIG. 16 can
further include a switch, a resistor, a capacitor, a transistor, a logic
circuit, or the like.

[0384] Next, structures of the light-emitting element will be described
with reference to FIGS. 17A to 17C. Here, a cross-sectional structure of
a pixel will be described by taking an n-channel driver TFT as an
example. Light-emitting element driving TFTs 7001, 7011, and 7021 used
for semiconductor devices illustrated in FIGS. 17A to 17C can be
manufactured in a manner similar to that of the thin film transistors
described in Embodiments 1 and 2 and are highly reliable thin film
transistors each including an oxide semiconductor layer.

[0385] In order to extract light emitted from the light-emitting element,
at least one of an anode and a cathode is required to transmit light. A
thin film transistor and a light-emitting element are formed over a
substrate. A light-emitting element can have a top emission structure in
which light emission is extracted through a surface opposite to the
substrate; a bottom emission structure in which light emission is
extracted through a surface on the substrate side; or a dual emission
structure in which light emission is extracted through the surface
opposite to the substrate and the surface on the substrate side. The
pixel configuration can be applied to a light-emitting element having any
of these emission structures.

[0386] A light-emitting element having a bottom emission structure will be
described with reference to FIG. 17A.

[0387]FIG. 17A is a cross-sectional view of a pixel in the case where the
light-emitting element driving TFT 7011 is an n-channel TFT and light is
emitted from a light-emitting element 7012 to a cathode 7013 side. In
FIG. 17A, the cathode 7013 of the light-emitting element 7012 is formed
over a light-transmitting conductive film 7017 which is electrically
connected to the light-emitting element driving TFT 7011, and an EL layer
7014 and an anode 7015 are stacked in this order over the cathode 7013.
Further, an insulating layer 7031 is formed over a substrate, an
insulating layer 7032 and an insulating layer 7036 are formed over a gate
electrode layer of the light-emitting element driving TFT 7011, and
insulating layers 7037, 7038, and 7039 are formed over a source electrode
layer and a drain electrode layer of the light-emitting element driving
TFT 7011. A source wiring 7018a and a source wiring 7018b are formed over
the insulating layer 7038 and connected to the source electrode layer of
the light-emitting element driving TFT 7011 through a contact hole formed
in the insulating layer 7037 and the insulating layer 7038. Note that the
light-transmitting conductive film 7017 is electrically connected to the
drain electrode of the light-emitting element driving TFT 7011 through a
contact hole formed in the insulating layers 7037, 7038, and 7039.

[0388] As the light-transmitting conductive film 7017, a
light-transmitting conductive film such as a film of indium oxide
including tungsten oxide, indium zinc oxide including tungsten oxide,
indium oxide including titanium oxide, indium tin oxide including
titanium oxide, indium tin oxide (hereinafter referred to as ITO), indium
zinc oxide, or indium tin oxide to which silicon oxide is added can be
used.

[0389] The cathode 7013 can be formed using various materials, and a
material having a low work function, for example, an alkali metal such as
Li or Cs, an alkaline earth metal such as Mg, Ca, or Sr, an alloy
including any of these (such as Mg:Ag or Al:Li), a rare earth metal such
as Yb or Er, or the like is preferable. In FIG. 17A, the thickness of the
cathode 7013 is approximately a thickness that transmits light
(preferably approximately 5 nm to 30 nm). For example, an aluminum film
having a thickness of 20 nm is used for the cathode 7013.

[0390] Note that the light-transmitting conductive film and the aluminum
film may be stacked and selectively etched to form the light-transmitting
conductive film 7017 and the cathode 7013; in this case, etching can be
performed using the same mask, which is preferable.

[0391] The peripheral portion of the cathode 7013 is covered with a
partition wall 7019. The partition wall 7019 is formed using an organic
resin film of polyimide, acrylic, polyamide, epoxy, or the like; an
inorganic insulating film; or organic polysiloxane. It is particularly
preferable that the partition wall 7019 be formed using a photosensitive
resin material to have an opening over the cathode 7013 so that a
sidewall of the opening is formed as an inclined surface with continuous
curvature. In the case where a photosensitive resin material is used for
the partition wall 7019, a step of forming a resist mask can be omitted.

[0392] The EL layer 7014 formed over the cathode 7013 and the partition
wall 7019 may be formed as a single layer or a plurality of layers
stacked. When the EL layer 7014 is formed as a plurality of layers, an
electron-injection layer, an electron-transport layer, a light-emitting
layer, a hole-transport layer, and a hole-injection layer are stacked in
this order over the cathode 7013. Note that not all of these layers need
to be provided.

[0393] The stacking order is not limited to the above stacking order, and
a hole-injection layer, a hole-transport layer, a light-emitting layer,
an electron-transport layer, and an electron-injection layer may be
stacked in this order over the cathode 7013. However, when power
consumption is compared, an electron-injection layer, an
electron-transport layer, a light-emitting layer, a hole-transport layer,
and a hole-injection layer are preferably stacked in this order over the
cathode 7013 because of lower power consumption.

[0394] For the anode 7015 formed over the EL layer 7014, various materials
can be used, and a material having a high work function such as titanium
nitride, ZrN, Ti, W, Ni, Pt, or Cr; or a light-transmitting conductive
material such as ITO, IZO (indium oxide zinc oxide), or ZnO is preferably
used, for example. For a light-blocking film 7016 over the anode 7015,
for example, a metal which blocks light, a metal which reflects light, or
the like is used. In this embodiment, an ITO film is used for the anode
7015, and a Ti film is used for the light-blocking film 7016.

[0395] The light-emitting element 7012 corresponds to a region where the
EL layer 7014 is sandwiched between the cathode 7013 and the anode 7015.
In the case of the element structure illustrated in FIG. 17A, light is
emitted from the light-emitting element 7012 to the cathode 7013 side as
indicated by an arrow.

[0396] Note that an example in which a light-transmitting conductive film
is used as a gate electrode layer is illustrated in FIG. 17A, and light
is emitted from the light-emitting element 7012 through a color filter
layer 7033.

[0397] The color filter layer 7033 is formed by a droplet discharge method
such as an inkjet method, a printing method, an etching method using a
photolithography technique, or the like.

[0398] The color filter layer 7033 is covered with an overcoat layer 7034,
and also covered with a protective insulating layer 7035. Note that the
overcoat layer 7034 with a thin thickness is illustrated in FIG. 17A;
however, the overcoat layer 7034 has a function to planarize a surface
with unevenness due to the color filter layer 7033.

[0399] A contact hole which is formed in the protective insulating layer
7035, the overcoat layer 7034, and insulating layers 7037, 7038, and 7039
and reaches the drain electrode layer is provided in a position
overlapping with the partition wall 7019. In FIG. 17A, the contact hole
which reaches the drain electrode layer 7030 and the partition wall 7019
overlap with each other, whereby the aperture ratio can be improved.

[0400] Next, a light-emitting element having a dual emission structure
will be described with reference to FIG. 17B.

[0401] In FIG. 17B, a cathode 7023 of a light-emitting element 7022 is
formed over a light-transmitting conductive film 7027 which is
electrically connected to the light-emitting element driving TFT 7021,
and an EL layer 7024 and an anode 7025 are stacked in this order over the
cathode 7023. Further, an insulating layer 7041 is formed over a
substrate, an insulating layer 7042 and an insulating layer 7046 are
formed over a gate electrode layer of the light-emitting element driving
TFT 7021, and insulating layers 7047, 7048, and 7049 are formed over a
source electrode layer and a drain electrode layer of the light-emitting
element driving TFT 7021. A source wiring 7028a and a source wiring 7028b
are formed over the insulating layer 7048 and connected to the source
electrode layer of the light-emitting element driving TFT 7021 through a
contact hole formed in the insulating layer 7047 and the insulating layer
7048. Note that the light-transmitting conductive film 7027 is
electrically connected to the drain electrode layer of the light-emitting
element driving TFT 7021 through a contact hole formed in the insulating
layers 7047, 7048, and 7049.

[0402] For the light-transmitting conductive film 7027, a
light-transmitting conductive film of indium oxide including tungsten
oxide, indium zinc oxide including tungsten oxide, indium oxide including
titanium oxide, indium tin oxide including titanium oxide, indium tin
oxide (hereinafter referred to as ITO), indium zinc oxide, indium tin
oxide to which silicon oxide is added, or the like can be used.

[0403] The cathode 7023 can be formed using a variety of materials, and a
material having a low work function, for example, an alkali metal such as
Li or Cs; an alkaline earth metal such as Mg, Ca, or Sr; an alloy
including any of these (such as Mg:Ag or Al:Li); a rare earth metal such
as Yb or Er; or the like is preferable. In this embodiment, the thickness
of the cathode 7023 is approximately a thickness that transmits light
(preferably approximately 5 nm to 30 nm). For example, an aluminum film
having a thickness of 20 nm is used for the cathode 7023.

[0404] Note that the light-transmitting conductive film and the aluminum
film may be stacked and selectively etched to form the light-transmitting
conductive film 7027 and the cathode 7023. In this case, etching can be
performed using the same mask, which is preferable.

[0405] The peripheral portion of the cathode 7023 is covered with a
partition wall 7029. The partition wall 7029 is formed using an organic
resin film of polyimide, acrylic, polyamide, epoxy, or the like; an
inorganic insulating film; or organic polysiloxane. It is particularly
preferable that the partition wall 7029 be formed using a photosensitive
resin material to have an opening over the cathode 7023 so that a
sidewall of the opening is formed as an inclined surface with continuous
curvature. In the case where a photosensitive resin material is used for
the partition wall 7029, a step of forming a resist mask can be omitted.

[0406] The EL layer 7024 formed over the cathode 7023 and the partition
wall 7029 may be formed as a single layer or a plurality of layers
stacked. When the EL layer 7024 is formed as a plurality of layers, an
electron-injection layer, an electron-transport layer, a light-emitting
layer, a hole-transport layer, and a hole-injection layer are stacked in
this order over the cathode 7023. Note that not all of these layers need
to be provided.

[0407] The stacking order is not limited to the above stacking order, and
a hole-injection layer, a hole-transport layer, a light-emitting layer,
an electron-transport layer, and an electron-injection layer may be
stacked in this order over the cathode 7023. However, when power
consumption is compared, an electron-injection layer, an
electron-transport layer, a light-emitting layer, a hole-transport layer,
and a hole-injection layer are preferably stacked in this order over the
cathode 7023 because of lower power consumption.

[0408] For the anode 7025 formed over the EL layer 7024, various materials
can be used, and a material having a high work function, for example, a
light-transmitting conductive material such as ITO, IZO, or ZnO is
preferable. In this embodiment, an ITO film including silicon oxide is
used for the anode 7025.

[0409] The light-emitting element 7022 corresponds to a region where the
EL layer 7024 is sandwiched between the cathode 7023 and the anode 7025.
In the case of the element structure illustrated in FIG. 17B, light is
emitted from the light-emitting element 7022 to both the anode 7025 side
and the cathode 7023 side as indicated by arrows.

[0410] Note that an example in which a light-transmitting conductive film
is used as the gate electrode layer is illustrated in FIG. 17B, and light
is emitted from the light-emitting element 7022 to the cathode 7023 side
through a color filter layer 7043.

[0411] The color filter layer 7043 is formed by a droplet discharge method
such as an inkjet method, a printing method, an etching method using a
photolithography technique, or the like.

[0412] The color filter layer 7043 is covered with an overcoat layer 7044,
and also covered with a protective insulating layer 7045.

[0413] A contact hole which is formed in the protective insulating layer
7045, the overcoat layer 7044, and the insulating layers 7047, 7048, and
7049 and reaches the drain electrode layer is provided in a position
overlapping with the partition wall 7029. The contact hole which reaches
the drain electrode layer and the partition wall 7029 overlap with each
other, whereby the aperture ratio on the anode 7025 side can be
substantially the same as the aperture ratio on the cathode 7023 side.

[0414] A contact hole which is formed in the protective insulating layer
7045 and the insulating layer 7042 and reaches the light-transmitting
conductive film 7027 is provided in a position overlapping with the
partition wall 7029.

[0415] Note that when a light-emitting element having a dual emission
structure is used and full color display is performed on both display
surfaces, light from the anode 7025 side does not pass through the color
filter layer 7043; therefore, a sealing substrate provided with another
color filter layer is preferably provided over the anode 7025.

[0416] Next, a light-emitting element having a top emission structure will
be described with reference to FIG. 17c.

[0417]FIG. 17c is a cross-sectional view of a pixel in the case where the
light-emitting element driving TFT 7001 is an n-channel TFT and light is
emitted from a light-emitting element 7002 to an anode 7005 side. In FIG.
17C, a cathode 7003 of the light-emitting element 7002 which is
electrically connected to the light-emitting element driving TFT 7001 is
formed, and an EL layer 7004 and the anode 7005 are stacked in this order
over the cathode 7003. Further, an insulating layer 7051 is formed over a
substrate, an insulating layer 7052 and an insulating layer 7056 are
formed over a gate electrode layer of the light-emitting element driving
TFT 7001, and insulating layers 7057, 7058, and 7059 are formed over a
source electrode layer and a drain electrode layer of the light-emitting
element driving TFT 7001. A source wiring 7008a and a source wiring 7008b
are formed over the insulating layer 7058 and connected to the source
electrode layer of the light-emitting element driving TFT 7001 through a
contact hole formed in the insulating layer 7057 and the insulating layer
7058. Note that the cathode 7003 is electrically connected to the drain
electrode layer of the light-emitting element driving TFT 7001 through
the contact hole formed in the insulating layers 7057, 7058, and 7059.

[0418] The cathode 7003 can be formed using a variety of materials, and a
material having a low work function, for example, an alkali metal such as
Li or Cs; an alkaline earth metal such as Mg, Ca, or Sr; an alloy
including any of these (such as Mg:Ag or Al:Li); a rare earth metal such
as Yb or Er; or the like is preferable.

[0419] The peripheral portion of the cathode 7003 is covered with a
partition wall 7009. The partition wall 7009 is formed using an organic
resin film of polyimide, acrylic, polyamide, epoxy, or the like; an
inorganic insulating film; or organic polysiloxane. It is particularly
preferable that the partition wall 7009 be formed using a photosensitive
resin material to have an opening over the cathode 7003 so that a
sidewall of the opening is formed as an inclined surface with continuous
curvature. In the case where a photosensitive resin material is used for
the partition wall 7009, a step of forming a resist mask can be omitted.

[0420] The EL layer 7004 which is formed over the cathode 7003 and the
partition wall 7009 may be formed using a single layer or a plurality of
layers stacked. When the EL layer 7004 is formed using a plurality of
layers, an electron-injection layer, an electron-transport layer, a
light-emitting layer, a hole-transport layer, and a hole-injection layer
are stacked in this order over the cathode 7003. Note that not all of
these layers need to be provided.

[0421] The stacking order is not limited to the above stacking order, and
a hole-injection layer, a hole-transport layer, a light-emitting layer,
an electron-transport layer, and an electron-injection layer may be
stacked in this order over the cathode 7003. In the case where these
layers are stacked in this order, the cathode 7003 functions as an anode.

[0422] In FIG. 17c, a hole-injection layer, a hole-transport layer, a
light-emitting layer, an electron-transport layer, and an
electron-injection layer are stacked in this order over a stacked film in
which a Ti film, an aluminum film, and a Ti film are formed in this
order, and thereover, a stacked layer of a Mg:Ag alloy thin film and an
ITO film is formed.

[0423] However, when power consumption is compared, an electron-injection
layer, an electron-transport layer, a light-emitting layer, a
hole-transport layer, and a hole-injection layer are preferably stacked
in this order over the cathode 7003 because of lower power consumption.

[0424] The anode 7005 is formed using a light-transmitting conductive
material which transmits light, and for example, a light-transmitting
conductive film of indium oxide including tungsten oxide, indium zinc
oxide including tungsten oxide, indium oxide including titanium oxide,
indium tin oxide including titanium oxide, indium tin oxide, indium zinc
oxide, indium tin oxide to which silicon oxide is added, or the like may
be used.

[0425] The light-emitting element 7002 corresponds to a region where the
EL layer 7004 is sandwiched between the cathode 7003 and the anode 7005.
In the case of the pixel illustrated in FIG. 17c, light is emitted from
the light-emitting element 7002 to the anode 7005 side as indicated by
arrows.

[0426] In FIG. 17c, the drain electrode layer of the light-emitting
element driving TFT 7001 is electrically connected to the cathode 7003
through a contact hole formed in the insulating layers 7057, 7058, and
7059. A planarization insulating layer 7053 can be formed using a resin
material such as polyimide, acrylic, benzocyclobutene, polyamide, or
epoxy. In addition to such resin materials, it is also possible to use a
low-dielectric constant material (a low-k material), a siloxane-based
resin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or
the like. Note that the planarization insulating layer 7053 may be formed
by stacking a plurality of insulating films formed using any of these
materials. There is no particular limitation on the method for forming
the planarization insulating layer 7053, and the planarization insulating
layer 7053 can be formed, depending on the material, by a method such as
a sputtering method, an SOG method, spin coating, dipping, spray coating,
or a droplet discharge method (such as an inkjet method, screen printing,
or offset printing), or with a tool such as a doctor knife, a roll
coater, a curtain coater, or a knife coater.

[0427] The partition wall 7009 is provided so as to insulate the cathode
7003 and a cathode of an adjacent pixel. The partition wall 7009 is
formed using an organic resin film of polyimide, acrylic, polyamide,
epoxy, or the like; an inorganic insulating film; or organic
polysiloxane. It is particularly preferable that the partition wall 7009
be formed using a photosensitive resin material to have an opening over
the cathode 7003 so that a sidewall of the opening is formed as an
inclined surface with continuous curvature. In the case where a
photosensitive resin material is used for the partition wall 7009, a step
of forming a resist mask can be omitted.

[0428] In the structure of FIG. 17c, when full color display is performed,
for example, the light-emitting element 7002 is used as a green
light-emitting element, one of adjacent light-emitting elements is used
as a red light-emitting element, and the other is used as a blue
light-emitting element. Alternatively, a light-emitting display device
capable of full color display may be manufactured using four kinds of
light-emitting elements, which include white light-emitting elements as
well as three kinds of light-emitting elements.

[0429] In the structure of FIG. 17c, a light-emitting display device
capable of full color display may be manufactured in such a manner that
all of a plurality of light-emitting elements which is arranged is white
light-emitting elements and a sealing substrate having a color filter or
the like is provided over the light-emitting element 7002. A material
which exhibits a single color such as white is formed and combined with a
color filter or a color conversion layer, whereby full color display can
be performed.

[0430] Needless to say, display of monochromatic light emission may be
performed. For example, a lighting device may be formed with the use of
white light emission, or an area-color light-emitting device may be
formed with the use of a single color light emission.

[0431] If necessary, an optical film such as a polarizing film including a
circularly polarizing plate may be provided.

[0432] Although an organic EL element is described here as a
light-emitting element, an inorganic EL element can also be provided as a
light-emitting element.

[0433] Note that the example is described in which a thin film transistor
(a light-emitting element driving TFT) which controls the driving of a
light-emitting element is electrically connected to the light-emitting
element; however, a structure may be employed in which a TFT for current
control is connected between the light-emitting element driving TFT and
the light-emitting element.

[0434] When a light-emitting element and a partition wall are not
provided, one embodiment of the present invention can also be applied to
a liquid crystal display device. The case of a liquid crystal display
device will be described in FIG. 37.

[0435] The case where a light-emitting element driving TFT 7061 is an
n-channel TFT is described. In FIG. 37, a light-transmitting conductive
film 7067 which is electrically connected to the light-emitting element
driving TFT 7061 is provided. Further, an insulating layer 7071 is formed
over a substrate, an insulating layer 7072 and an insulating layer 7076
are formed over a gate electrode layer of the light-emitting element
driving TFT 7061, and insulating layers 7077, 7078, and 7079 are formed
over a source electrode layer and a drain electrode layer of the
light-emitting element driving TFT 7061. A source wiring 7068a and a
source wiring 7068b are formed over the insulating layers 7077 and 7078
and connected to the source electrode layer of the light-emitting element
driving TFT 7061 through a contact hole formed in the insulating layer
7077 and the insulating layer 7078. The light-transmitting conductive
film 7067 is electrically connected to the drain electrode layer of the
light-emitting element driving TFT 7061 through a contact hole formed in
the insulating layers 7077, 7078, and 7079.

[0436] As the light-transmitting conductive film 7067, a
light-transmitting conductive film of indium oxide including tungsten
oxide, indium zinc oxide including tungsten oxide, indium oxide including
titanium oxide, indium tin oxide including titanium oxide, indium tin
oxide (hereinafter referred to as ITO), indium zinc oxide, indium tin
oxide to which silicon oxide is added, or the like can be used.

[0437] Note that in FIG. 37, light is emitted from a backlight or the like
through a color filter layer 7063. The color filter layer 7063 is formed
by a droplet discharge method such as an inkjet method, a printing
method, an etching method using a photolithography technique, or the
like.

[0438] The color filter layer 7063 is covered with an overcoat layer 7064,
and also covered with a protective insulating layer 7065. Note that the
overcoat layer 7064 with a thin thickness is illustrated in FIG. 37;
however, the overcoat layer 7064 has a function to planarize a surface
with unevenness due to the color filter layer 7063.

[0439] A structure in which a liquid crystal layer is provided over the
light-transmitting conductive film 7067 can be applied to a liquid
crystal display device.

[0440] Next, the appearance and a cross section of a light-emitting
display panel (also referred to as a light-emitting panel), which is one
embodiment of the semiconductor device, will be described with reference
to FIGS. 15A and 15B. FIG. 15A is a plan view of a panel in which a thin
film transistor and a light-emitting element formed over a first
substrate are sealed between the first substrate and a second substrate
with a sealant. FIG. 15B is a cross-sectional view taken along line H-I
of FIG. 15A.

[0441] A sealant 4505 is provided to surround a pixel portion 4502, a
signal line driver circuit 4503a, a signal line driver circuit 4503b, a
scan line driver circuit 4504a, and a scan line driver circuit 4504b,
which are provided over a first substrate 4501. In addition, a second
substrate 4506 is provided over the pixel portion 4502, the signal line
driver circuits 4503a and 4503b, and the scan line driver circuits 4504a
and 4504b. Accordingly, the pixel portion 4502, the signal line driver
circuits 4503a and 4503b, and the scan line driver circuits 4504a and
4504b are sealed together with a filler 4507, by the first substrate
4501, the sealant 4505, and the second substrate 4506. It is preferable
that a display device be thus packaged (sealed) with a protective film
(such as a bonding film or an ultraviolet curable resin film) or a cover
material with high air-tightness and little degasification so that the
display device is not exposed to the outside air.

[0442] The pixel portion 4502, the signal line driver circuits 4503a and
4503b, and the scan line driver circuits 4504a and 4504b formed over the
first substrate 4501 each include a plurality of thin film transistors,
and a thin film transistor 4510 included in the pixel portion 4502 and a
thin film transistor 4509 included in the signal line driver circuit
4503a are illustrated as an example in FIG. 15B. Insulating layers 4541,
4542, and 4543 are provided over the thin film transistors 4509 and 4510.
An insulating layer 4544 is provided over the thin film transistor 4510.
Further, an insulating layer 4545 is provided over the first substrate
4501, and an insulating layer 4546 and an insulating layer 4547 are
provided over gate electrode layers of the thin film transistors. A
source wiring 4548 is provided over the insulating layer 4542 and
connected to a source electrode layer or a drain electrode layer of the
thin film transistor 4510 through a contact hole formed in the insulating
layer 4541 and the insulating layer 4542.

[0443] As the thin film transistors 4509 and 4510, the highly reliable
thin film transistor including an oxide semiconductor layer, which is
described in any of Embodiments 1 to 3, can be employed. In this
embodiment, the thin film transistors 4509 and 4510 are re-channel thin
film transistors.

[0444] A conductive layer 4540 is provided over an insulating layer 4543
so as to overlap with a channel formation region of the oxide
semiconductor layer of the thin film transistor 4509 for the driver
circuit. When the conductive layer 4540 is provided in a position
overlapping with the channel formation region of the oxide semiconductor
layer, the amount of shift in the threshold voltage of the thin film
transistor 4509 before and after a BT test can be reduced. The conductive
layer 4540 may have a potential which is the same as or different from
that of the gate electrode layer of the thin film transistor 4509, and
can function as a second gate electrode layer. The potential of the
conductive layer 4540 may be GND or 0 V, or the conductive layer 4540 may
be in a floating state.

[0445] In the thin film transistors 4509, the insulating layer 4541 is
formed as a protective insulating film to be in contact with a
semiconductor layer including channel formation region. The insulating
layer 4541 may be formed using a material and a method similar to those
of the insulating layer 208 described in Embodiment 1. In addition, in
order to reduce surface roughness of a thin film transistor, the thin
film transistor 4510 is covered with the insulating layer 4544
functioning as a planarization insulating film. Here, a silicon oxide
film is formed as the insulating layer 4541 by a sputtering method in a
manner similar to that of the insulating layer 208 described in
Embodiment 1.

[0446] Further, the insulating layer 4544 is formed as the planarization
insulating film. The insulating layer 4544 may be formed using a material
and a method similar to those of the insulating layer 4021 described in
Embodiment 7. Here, acrylic is used for the insulating layer 4544 serving
as a planarization insulating layer.

[0447] Reference numeral 4511 denotes a light-emitting element, and a
first electrode layer 4517 that is a pixel electrode included in the
light-emitting element 4511 is electrically connected to the source
electrode layer or the drain electrode layer of the thin film transistor
4510. Note that a structure of the light-emitting element 4511 is not
limited to the structure, which is a stacked-layer structure of the first
electrode layer 4517, an electroluminescent layer 4512, and a second
electrode layer 4513. The structure of the light-emitting element 4511
can be changed as appropriate, depending on the direction in which light
is extracted from the light-emitting element 4511, or the like.

[0448] A partition wall 4520 is formed using an organic resin film, an
inorganic insulating film, or organic polysiloxane. It is particularly
preferable that the partition wall 4520 be formed using a photosensitive
material to have an opening over the first electrode layer 4517 so that a
sidewall of the opening is formed as an inclined surface with continuous
curvature.

[0449] The electroluminescent layer 4512 may be formed as a single layer
or a plurality of layers stacked.

[0450] A protective film may be formed over the second electrode layer
4513 and the partition wall 4520 in order to prevent oxygen, hydrogen,
moisture, carbon dioxide, or the like from entering the light-emitting
element 4511. As the protective film, a silicon nitride film, a silicon
nitride oxide film, a DLC film, or the like can be formed.

[0451] A variety of signals and potentials are supplied to the signal line
driver circuits 4503a and 4503b, the scan line driver circuits 4504a and
4504b, or the pixel portion 4502 from an FPC 4518a and an FPC 4518b.

[0452] A connection terminal electrode 4515 is formed from the same
conductive film as the first electrode layer 4517 included in the
light-emitting element 4511, and a terminal electrode 4516 is formed from
the same conductive film as source electrode layers and drain electrode
layers included in the thin film transistors 4509 and 4510.

[0453] The connection terminal electrode 4515 is electrically connected to
a terminal of the FPC 4518a through an anisotropic conductive film 4519.

[0454] The second substrate located in the direction in which light is
extracted from the light-emitting element 4511 needs to have a
light-transmitting property. In that case, a light-transmitting material
such as a glass plate, a plastic plate, a polyester film, or an acrylic
film is used.

[0455] As the filler 4507, an ultraviolet curable resin or a thermosetting
resin can be used, in addition to an inert gas such as nitrogen or argon.
For example, polyvinyl chloride (PVC), acrylic, polyimide, epoxy resin,
silicone resin, polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA)
can be used. For example, nitrogen may be used for the filler.

[0456] If needed, an optical film such as a polarizing plate, a circularly
polarizing plate (including an elliptically polarizing plate), a
retardation plate (a quarter-wave plate or a half-wave plate), or a color
filter may be provided as appropriate on a light-emitting surface of the
light-emitting element. Furthermore, the polarizing plate or the
circularly polarizing plate may be provided with an anti-reflection film.
For example, anti-glare treatment by which reflected light can be
diffused by projections and depressions on the surface so as to reduce
the glare can be performed.

[0457] The signal line driver circuits 4503a and 4503b and the scan line
driver circuits 4504a and 4504b may be mounted as driver circuits formed
using a single crystal semiconductor film or a polycrystalline
semiconductor film over a substrate separately prepared. Alternatively,
only the signal line driver circuits or part thereof, or only the scan
line driver circuits or part thereof may be separately formed and
mounted. This embodiment is not limited to the structure illustrated in
FIGS. 15A and 15B.

[0458] Through the above process, a highly reliable light-emitting display
device (display panel) as a semiconductor device can be manufactured.

[0459] By manufacturing the above light-emitting display device with the
use of the display device described in any of Embodiments 1 to 5, a gate
wiring or a source wiring can be formed using a conductive material
including Cu; accordingly, increase in wiring resistance can be
prevented. Consequently, high speed operation and low power consumption
of the above light-emitting display device can be achieved, and thus the
light-emitting display device can have a large-sized screen or a high
definition screen.

[0460] This embodiment can be combined with any of the structures
described in the other embodiments as appropriate.

Embodiment 10

[0461] A semiconductor device disclosed in this specification can be
applied to electronic paper. Electronic paper can be used for electronic
appliances in a variety of fields as long as they can display data. For
example, electronic paper can be applied to an electronic book reader (an
e-book reader), a poster, an advertisement in a vehicle such as a train,
or displays of various cards such as a credit card. Examples of such
electronic appliances are illustrated in FIGS. 24A and 24B and FIG. 25.

[0462]FIG. 24A illustrates a poster 2631 using electronic paper. In the
case where an advertising medium is printed paper, the advertisement is
replaced by hands; however, by using the electronic paper, the
advertising display can be changed in a short time. Furthermore, stable
images can be obtained without display defects. Note that the poster may
have a configuration capable of wirelessly transmitting and receiving
data.

[0463] By manufacturing the poster 2631 with the use of the display device
described in any of Embodiments 1 to 5, a gate wiring or a source wiring
can be formed using a conductive material including Cu; accordingly,
increase in wiring resistance can be prevented. Consequently, high speed
operation and low power consumption of the above display device can be
achieved, and thus the poster 2631 can have a large-sized screen or a
high definition screen.

[0464] FIG. 24B illustrates an advertisement 2632 in a vehicle such as a
train. In the case where an advertising medium is printed paper, the
advertisement is replaced by hands; however, by using the electronic
paper, much manpower is not needed and the advertising display can be
changed in a short time. Furthermore, stable images can be obtained
without display defects. Note that the advertisement in a vehicle may
have a configuration capable of wirelessly transmitting and receiving
data.

[0465] By manufacturing the advertisement 2632 in a vehicle with the use
of the display device described in any of Embodiments 1 to 5, a gate
wiring or a source wiring can be formed using a conductive material
including Cu; accordingly, increase in wiring resistance can be
prevented. Consequently, high speed operation and low power consumption
of the above display device can be achieved, and thus the advertisement
2632 in a vehicle can have a large-sized screen or a high definition
screen.

[0466]FIG. 25 illustrates an example of an electronic book reader. For
example, an electronic book reader 2700 includes two housings, a housing
2701 and a housing 2703. The housing 2701 and the housing 2703 are
combined with a hinge 2711 so that the electronic book reader 2700 can be
opened and closed with the hinge 2711 as an axis. With such a structure,
the electronic book reader 2700 can operate like a paper book.

[0467] A display portion 2705 and a display portion 2707 are incorporated
in the housing 2701 and the housing 2703, respectively. The display
portion 2705 and the display portion 2707 may display one image or
different images. In the case where the display portion 2705 and the
display portion 2707 display different images, for example, text can be
displayed on a display portion on the right side (the display portion
2705 in FIG. 25) and graphics can be displayed on a display portion on
the left side (the display portion 2707 in FIG. 25).

[0468]FIG. 25 illustrates an example in which the housing 2701 is
provided with an operation portion and the like. For example, the housing
2701 is provided with a power switch 2721, an operation key 2723, a
speaker 2725, and the like. With the operation key 2723, pages can be
turned. Note that a keyboard, a pointing device, and the like may be
provided on a surface of the housing, on which the display portion is
provided. Furthermore, an external connection terminal (such as an
earphone terminal, a USB terminal, or a terminal that can be connected to
various cables like an AC adapter and a USB cable), a recording medium
insertion portion, and the like may be provided on the back surface or a
side surface of the housing. Moreover, the electronic book reader 2700
may have a function of an electronic dictionary.

[0469] The electronic book reader 2700 may have a configuration capable of
wirelessly transmitting and receiving data. Through wireless
communication, desired book data or the like can be purchased and
downloaded from an electronic book server.

Embodiment 11

[0470] A semiconductor device disclosed in this specification can be
applied to a variety of electronic appliances (including amusement
machines). Examples of electronic appliances include television sets
(also referred to as televisions or television receivers), monitors of
computers or the like, cameras such as digital cameras and digital video
cameras, digital photo frames, cellular phones (also referred to as
mobile phones or mobile phone sets), portable game consoles, portable
information terminals, audio reproducing devices, large-sized game
machines such as pachinko machines, and the like.

[0471]FIG. 26A illustrates an example of a television set. In a
television set 9600, a display portion 9603 is incorporated in a housing
9601. Images can be displayed on the display portion 9603. Here, the
housing 9601 is supported by a stand 9605.

[0472] The television set 9600 can be operated with an operation switch of
the housing 9601 or a separate remote controller 9610. Channels and
volume can be controlled with an operation key 9609 of the remote
controller 9610 so that an image displayed on the display portion 9603
can be controlled. Furthermore, the remote controller 9610 may be
provided with a display portion 9607 for displaying data output from the
remote controller 9610.

[0473] Note that the television set 9600 is provided with a receiver, a
modem, and the like. With the receiver, a general television broadcast
can be received. Furthermore, when the television set 9600 is connected
to a communication network by wired or wireless connection via the modem,
one-way (from a transmitter to a receiver) or two-way (between a
transmitter and a receiver, between receivers, or the like) data
communication can be performed.

[0474] By manufacturing the television set 9600 with the use of the
display device described in any of Embodiments 1 to 5, a gate wiring or a
source wiring can be formed using a conductive material including Cu;
accordingly, increase in wiring resistance can be prevented.
Consequently, high speed operation and low power consumption of the above
display device can be achieved, and thus the television set 9600 can have
a large-sized screen or a high definition screen.

[0475]FIG. 26B illustrates an example of a digital photo frame. For
example, in a digital photo frame 9700, a display portion 9703 is
incorporated in a housing 9701. Various images can be displayed on the
display portion 9703. For example, data of an image taken by a digital
camera or the like can be displayed, and the digital photo frame can
function as a normal photo frame.

[0476] Note that the digital photo frame 9700 is provided with an
operation portion, an external connection portion (such as a USB terminal
or a terminal that can be connected to various cables like a USB cable),
a recording medium insertion portion, and the like.

[0477] Although these components may be provided on a surface where the
display portion is provided, it is preferable to provide them on a side
surface or the back surface for the design of the digital photo frame
9700. For example, a memory storing data of an image taken by a digital
camera is inserted in the recording medium insertion portion of the
digital photo frame, whereby the image data can be downloaded and
displayed on the display portion 9703.

[0478] The digital photo frame 9700 may have a configuration capable of
wirelessly transmitting and receiving data. Through wireless
communication, desired image data can be downloaded to be displayed.

[0479]FIG. 27A illustrates a portable amusement machine including two
housings, a housing 9881 and a housing 9891. The housings 9881 and 9891
are connected with a connection portion 9893 so as to be opened and
closed. A display portion 9882 and a display portion 9883 are
incorporated in the housing 9881 and the housing 9891, respectively. In
addition, the portable amusement machine illustrated in FIG. 27A includes
a speaker portion 9884, a recording medium insertion portion 9886, an LED
lamp 9890, input means (an operation key 9885, a connection terminal
9887, a sensor 9888 (having a function of measuring force, displacement,
position, speed, acceleration, angular velocity, rotational frequency,
distance, light, liquid, magnetism, temperature, chemical substance,
sound, time, hardness, electric field, current, voltage, electric power,
radiation, flow rate, humidity, gradient, oscillation, odor, or infrared
rays), and a microphone 9889), and the like. Needless to say, the
structure of the portable amusement machine is not limited to the above
and other structures provided with at least a semiconductor device
disclosed in this specification can be employed. The portable amusement
machine may include other accessory equipment, as appropriate. The
portable amusement machine illustrated in FIG. 27A has a function of
reading a program or data stored in a recording medium to display it on
the display portion, and a function of sharing information with another
portable amusement machine by wireless communication. The portable
amusement machine illustrated in FIG. 27A can have various functions
without limitation to the above.

[0480]FIG. 27B illustrates an example of a slot machine which is a
large-sized amusement machine. In a slot machine 9900, a display portion
9903 is incorporated in a housing 9901. In addition, the slot machine
9900 includes an operation means such as a start lever or a stop switch,
a coin slot, a speaker, and the like. Needless to say, the structure of
the slot machine 9900 is not limited to the above and other structures
provided with at least a semiconductor device disclosed in this
specification may be employed. The slot machine 9900 may include other
accessory equipment, as appropriate.

[0481]FIG. 28A is a perspective view illustrating an example of a
portable computer.

[0482] In the portable computer of FIG. 28A, a top housing 9301 having a
display portion 9303 and a bottom housing 9302 having a keyboard 9304 can
overlap with each other by closing a hinge unit which connects the top
housing 9301 and the bottom housing 9302. The portable computer of FIG.
28A can be convenient for carrying, and in the case of using the keyboard
for input, the hinge unit is opened and the user can input looking at the
display portion 9303.

[0483] The bottom housing 9302 includes a pointing device 9306 with which
input can be performed, in addition to the keyboard 9304. Further, when
the display portion 9303 is a touch input panel, input can be performed
by touching part of the display portion. The bottom housing 9302 includes
an arithmetic function portion such as a CPU or hard disk. In addition,
the bottom housing 9302 includes an external connection port 9305 into
which another device such as a communication cable conformable to
communication standards of a USB is inserted.

[0484] The top housing 9301 further includes a display portion 9307 which
can be stored in the top housing 9301 by being slid therein. Thus, a
large display screen can be realized. In addition, the user can adjust
the orientation of a screen of the storable display portion 9307. When
the storable display portion 9307 is a touch input panel, input can be
performed by touching part of the storable display portion.

[0485] The display portion 9303 or the storable display portion 9307 is
formed using an image display device of a liquid crystal display panel, a
light-emitting display panel such as an organic light-emitting element or
an inorganic light-emitting element, or the like.

[0486] In addition, the portable computer of FIG. 28A, which can be
provided with a receiver or the like, can receive a television broadcast
to display an image on the display portion. While the hinge unit which
connects the top housing 9301 and the bottom housing 9302 is kept closed,
the whole screen of the display portion 9307 is exposed by sliding the
display portion 9307 out and the angle of the screen is adjusted; thus,
the user can watch a television broadcast. In this case, the hinge unit
is not opened and display is not performed on the display portion 9303.
In addition, start up of only a circuit which displays the television
broadcast is performed. Therefore, power consumption can be minimized,
which is advantageous for the portable computer whose battery capacity is
limited.

[0487]FIG. 28B is a perspective view illustrating an example of a
cellular phone that the user can wear on the wrist like a wristwatch.

[0488] This cellular phone is formed with a main body which includes a
communication device including at least a telephone function, and a
battery; a band portion 9204 which enables the main body to be worn on
the wrist; an adjusting portion 9205 for adjusting the fixation of the
band portion fixed for the wrist; a display portion 9201; a speaker 9207;
and a microphone 9208.

[0489] In addition, the main body includes operation switches 9203. The
operation switches 9203 serve, for example, as a switch for starting a
program for the Internet when the switch is pressed, in addition to
serving as a switch for turning on a power source, a switch for shifting
a display, a switch for instructing to start taking images, or the like,
and can be used so as to correspond to each function.

[0490] Input to this cellular phone is performed by touching the display
portion 9201 with a finger or an input pen, operating the operation
switches 9203, or inputting voice into the microphone 9208. Note that
displayed buttons 9202 which are displayed on the display portion 9201
are illustrated in FIG. 28B. Input can be performed by touching the
displayed buttons 9202 with a finger or the like.

[0491] Further, the main body includes a camera portion 9206 including an
image pick-up means having a function of converting an image of an
object, which is formed through a camera lens, to an electronic image
signal. Note that the camera portion is not necessarily provided.

[0492] The cellular phone illustrated in FIG. 28B is provided with a
receiver of a television broadcast or the like, and can display an image
on the display portion 9201 by receiving a television broadcast. In
addition, the cellular phone illustrated in FIG. 28B is provided with a
memory device and the like such as a memory, and can record a television
broadcast in the memory. The cellular phone illustrated in FIG. 28B may
have a function of collecting location information such as GPS.

[0493] An image display device of a liquid crystal display panel, a
light-emitting display panel such as an organic light-emitting element or
an inorganic light-emitting element, or the like is used as the display
portion 9201. The cellular phone illustrated in FIG. 28B is compact and
lightweight, and the battery capacity thereof is limited. Therefore, a
panel which can be driven with low power consumption is preferably used
as a display device for the display portion 9201.

[0494] Note that FIG. 28B illustrates the electronic appliance which is
worn on the wrist; however, this embodiment is not limited thereto as
long as a portable shape is employed.

Embodiment 12

[0495] In this embodiment, examples of display devices including the thin
film transistor described in Embodiment 1 will be described as one
embodiment of a semiconductor device with reference to FIG. 29, FIG. 30,
FIG. 31, and FIG. 32. In this embodiment, an example of a liquid crystal
display device using a liquid crystal element as a display element will
be described with reference to FIG. 29, FIG. 30, FIG. 31, and FIG. 32.
Any of the thin film transistors described in Embodiments 1 and 2 can be
applied to a TFT 628 and a TFT 629 used for liquid crystal display
devices illustrated in FIG. 29, FIG. 30, FIG. 31, and FIG. 32, and the
TFT 628 and the TFT 629 can be manufactured through a process similar to
that described in Embodiment 2 and have high electric characteristics and
high reliability. The TFT 628 and the TFT 629 are each a thin film
transistor including an oxide semiconductor layer as a channel formation
region. The case where the thin film transistor illustrated in FIGS. 1A
and 1B is used as an example of a thin film transistor will described
with reference to FIG. 29, FIG. 30, FIG. 31, and FIG. 32, but an
embodiment of the present invention is not limited thereto.

[0496] A vertical alignment (VA) liquid crystal display device is
described. The VA liquid crystal display device has a kind of form in
which alignment of liquid crystal molecules of a liquid crystal display
panel is controlled. The VA liquid crystal display device has a form in
which liquid crystal molecules are vertical to a panel surface when
voltage is not applied. In this embodiment, in particular, a pixel is
divided into some regions (subpixels), and molecules are aligned in
different directions in their respective regions. This is referred to as
domain multiplication or multi-domain design. In the following
description, a liquid crystal display device with multi-domain design is
described.

[0497]FIG. 30 and FIG. 31 illustrate a pixel electrode and a counter
electrode, respectively. FIG. 30 is a plan view of a substrate side,
which is provided with the pixel electrode. FIG. 29 illustrates a
cross-sectional structure taken along line E-F of FIG. 30. FIG. 31 is a
plan view of a substrate side, which is provided with the counter
electrode. Hereinafter, description is made with reference to these
drawings.

[0498]FIG. 29 illustrates a state in which a substrate 600 provided with
the TFT 628, a pixel electrode layer 624 electrically connected thereto,
and a storage capacitor portion 630 overlaps with a counter substrate 601
provided with a counter electrode layer 640 and the like, and liquid
crystal is injected therebetween.

[0499] Coloring films 636 (a first coloring film, a second coloring film,
and a third coloring film (not illustrated)) and the counter electrode
layer 640 are formed in a position where the counter substrate 601 is
provided with a spacer (not illustrated), and the projection 644 is
formed on the counter electrode layer 640 is provided. With this
structure, the height of the projection 644 for controlling alignment of
liquid crystal is made different from that of the spacer. An alignment
film 648 is formed over the pixel electrode layer 624. In a similar
manner, the counter electrode layer 640 and the projection 644 are
provided with an alignment film 646. A liquid crystal layer 650 is formed
between the substrate 600 and the counter substrate 601.

[0500] As the spacer, columnar spacer may be used or bead spacers may be
dispersed. Further, the spacer may also be formed over the pixel
electrode layer 624 provided over the substrate 600.

[0501] The TFT 628, the pixel electrode layer 624 electrically connected
thereto, and the storage capacitor portion 630 are formed over the
substrate 600 provided with an insulating layer 661. The pixel electrode
layer 624 is connected to a wiring 618 through a contact hole 623 which
penetrates an insulating layer 664 that covers the TFT 628, a source
wiring 616, and the storage capacitor portion 630, an insulating layer
665 over the insulating layer an insulating layer 666 over the insulating
layer 665, and an insulating layer 622 over the insulating layer 666.
Further, the source wiring 616 which includes a stack of a source wiring
616a and a source wiring 616b is formed over the insulating layer 665 and
connected to a source electrode layer or a drain electrode layer of the
TFT 628 through a contact hole formed in the insulating layer 665 and the
insulating layer 664. Here, the thin film transistor described in
Embodiment 1 can be used as the TFT 628 as appropriate.

[0502] The storage capacitor portion 630 includes a first capacitor wiring
604 which is formed at the same time as a gate wiring 602 of the TFT 628,
insulating layers 662 and 663 over the gate wiring 602, and a second
capacitor wiring 617 which is formed at the same time as the wiring 618.
Here, the gate wiring 602 is a stack of gate wirings 602a and 602b, and
the gate wiring 602b functions as a gate electrode layer of the TFT 628.
The capacitor wiring 604 is also a stack of capacitor wirings 604a and
604b.

[0503] A liquid crystal element is formed by overlapping of the pixel
electrode layer 624, the liquid crystal layer 650, and the counter
electrode layer 640.

[0504]FIG. 30 illustrates a plan structure over the substrate 600. The
pixel electrode layer 624 is formed using the material described in
Embodiment 1. The pixel electrode layer 624 is provided with a slit 625.
The slit 625 is provided to control alignment of the liquid crystal.

[0505] The TFT 629, a pixel electrode layer 626 electrically connected
thereto, and a storage capacitor portion 631 illustrated in FIG. 30 can
be formed in a similar manner to that of the TFT 628, the pixel electrode
layer 624, and the storage capacitor portion 630, respectively. Note that
a capacitor wiring 605 included in the storage capacitor portion 631 is
also a stack of capacitor wirings 605a and 605b, which is similar to the
case of the capacitor wiring 604. Here, the TFT 628 and the TFT 629 are
both connected to the source wiring 616 and the gate wiring 602. A pixel
of this liquid crystal display panel includes the pixel electrode layers
624 and 626. Each of the pixel electrode layers 624 and 626 is in a
sub-pixel.

[0506]FIG. 31 illustrates a plan structure on the counter substrate side.
The counter electrode layer 640 is preferably formed using a material
similar to that of the pixel electrode layer 624. The counter electrode
layer 640 is provided with the projection 644 for controlling alignment
of the liquid crystal. Note that in FIG. 31, the pixel electrode layers
624 and 626 formed over the substrate 600 are indicated by dashed lines,
and the counter electrode layer 640 overlaps with the pixel electrode
layers 624 and 626.

[0507]FIG. 32 illustrates an equivalent circuit of this pixel structure.
Both the TFT 628 and the TFT 629 are connected to the gate wiring 602 and
the source wiring 616. In this case, when potentials of the capacitor
wiring 604 and the capacitor wiring 605 are different from each other,
operations of a liquid crystal element 651 and a liquid crystal element
652 can be different from each other. That is, alignment of the liquid
crystal is precisely controlled and the viewing angle is increased by
individual control of potentials of the capacitor wirings 604 and 605.

[0508] When voltage is applied to the pixel electrode layer 624 provided
with the slit 625, electric field distortion (an oblique electric field)
is generated in the vicinity of the slit 625. The slit 625 and the
projection 644 on the counter substrate 601 side are alternately arranged
in an engaging manner, and thus an oblique electric field is effectively
generated to control alignment of the liquid crystal, so that a direction
of alignment of the liquid crystal varies depending on location. That is,
the viewing angle of the liquid crystal display panel is increased by
domain multiplication.

[0509] Next, another VA liquid crystal display device, which is different
from the above, is described with reference to FIG. 33, FIG. 34, FIG. 35,
and FIG. 36. In the structures of the invention to be given below,
portions which are the same as or have functions similar to those of the
above VA liquid crystal display device are denoted by the same reference
numerals in different drawings, and repetitive description thereof is
omitted.

[0510]FIG. 33 and FIG. 34 each illustrate a pixel structure of a VA
liquid crystal display panel. FIG. 34 is a plan view of the substrate
600. FIG. 33 illustrates a cross-sectional structure taken along line Y-Z
of FIG. 34.

[0511] In this pixel structure, a plurality of pixel electrodes is
included in one pixel, and each of the pixel electrodes is connected to a
respective TFT. Each TFT is driven by a different gate signal. That is,
this is a structure in which a signal supplied to each pixel electrode is
individually controlled in a multi-domain pixel.

[0512] Through the contact hole 623 penetrating the insulating layer 664,
the insulating layer 665, and the insulating layer 666, the pixel
electrode layer 624 is connected to the TFT 628 via the wiring 618.
Through a contact hole 627 penetrating the insulating layer 664, the
insulating layer 665, and the insulating layer 666, the pixel electrode
layer 626 is connected to the TFT 629 via a wiring 619. The gate wiring
602 of the TFT 628 and a gate wiring 603 of the TFT 629 are separated so
that different gate signals can be supplied thereto. Meanwhile, the
source wiring 616 which functions as a data line is connected to the
source electrode layers of the TFT 628 and the TFT 629 through contact
holes formed in the insulating layer 664 and the insulating layer 665,
and commonly used between the TFT 628 and the TFT 629. As each of the
TFTs 628 and 629, the thin film transistor described in Embodiment 1 can
be used as appropriate. A capacitor wiring 690 is also provided. Note
that similarly to the pixel structure of the above VA liquid crystal
display panel, the gate wiring 602 is a stack of the gate wirings 602a
and 602b, the gate wiring 603 is a stack of gate wirings 603a and 603b,
the source wiring 616 is a stack of the source wirings 616a and 616b, and
the capacitor wiring 690 is a stack of capacitor wirings 690a and 690b.
In addition, the insulating layers 661 to 666 are formed as those in the
pixel structure of the above VA liquid crystal display panel.

[0513] The pixel electrode layers 624 and 626 have different shapes and
are separated by the slit 625. The pixel electrode layer 626 is formed so
as to surround the pixel electrode layer 624 which has a V shape. Voltage
applied between the pixel electrode layers 624 and 626 is made to vary by
the TFTs 628 and 629 in order to control alignment of the liquid crystal.
FIG. 36 illustrates an equivalent circuit of this pixel structure. The
TFT 628 is connected to the gate wiring 602. The TFT 629 is connected to
the gate wiring 603. The TFTs 628 and 629 are both connected to the
source wiring 616. When different gate signals are supplied to the gate
wirings 602 and 603, the operations of the liquid crystal elements 651
and 652 can vary. In other words, when the operations of the TFTs 628 and
629 are controlled separately to precisely control the alignment of the
liquid crystal in the liquid crystal elements 651 and 652, the viewing
angle can be increased.

[0514] The counter substrate 601 is provided with the coloring film 636
and the counter electrode layer 640. Moreover, a planarization film 637
is formed between the coloring film 636 and the counter electrode layer
640 to prevent alignment disorder of the liquid crystal. FIG. 35
illustrates a plan structure on the counter substrate side. A slit 641 is
formed in the counter electrode layer 640, which is used in common
between different pixels. The slits 641 and 625 on the pixel electrode
layers 624 and 626 side are alternately arranged in an engaging manner;
thus, an oblique electric field is effectively generated, and alignment
of the liquid crystal can be controlled. Accordingly, the direction in
which the liquid crystal is aligned can vary depending on location, and
the viewing angle is increased. Note that in FIG. 32, the pixel electrode
layers 624 and 626 which are formed over the substrate 600 are indicated
by dashed lines, and the counter electrode layer 640 overlaps with the
pixel electrode layers 624 and 626.

[0515] The alignment film 648 is formed over the pixel electrode layer 624
and the pixel electrode layer 626. In a similar manner, the counter
electrode layer 640 is provided with the alignment film 646. The liquid
crystal layer 650 is formed between the substrate 600 and the counter
substrate 601. Further, the pixel electrode layer 624, the liquid crystal
layer 650, and the counter electrode layer 640 overlap with each other,
so that the liquid crystal element 651 is formed. The pixel electrode
layer 626, the liquid crystal layer 650, and the counter electrode layer
640 overlap with each other, so that the liquid crystal element 652 is
formed. The pixel structure of the display panel illustrated in FIG. 33
to FIG. 36 is a multi-domain structure in which the liquid crystal
element 651 and the liquid crystal element 652 are provided in one pixel.

[0516] With the use of the display device described in any of Embodiments
1 to 5, a liquid crystal display device like the above can be
manufactured. Although the vertical alignment (VA) liquid crystal display
device is described, this embodiment is not limited thereto. For example,
a liquid crystal display device in a horizontal electric field mode
(e.g., an IPS liquid crystal display device) in which a horizontal
electric field is applied to liquid crystal molecules in a cell, whereby
liquid crystal is driven to express gray scales or a TN liquid crystal
display device may be employed.

[0517] By manufacturing the above liquid crystal display device with the
use of the display device described in any of Embodiments 1 to 5, a gate
wiring or a source wiring can be formed using a conductive material
including Cu; accordingly, increase in wiring resistance can be
prevented. Consequently, high speed operation and low power consumption
of the display device can be achieved, and thus the liquid crystal
display device can have a large-sized screen or a high definition screen.

Embodiment 13

[0518] In this embodiment, an example of manufacturing a display panel in
which a first substrate provided with a thin film transistor and a second
substrate serving as a counter substrate are bonded to each other will be
described below.

[0519] In a production process of liquid crystal display panels or EL
display panels, static electricity might affect an electronic circuit
adversely, which results in variation in electric characteristics or
breakdown of the circuit. In addition, there is a problem in that static
electricity easily causes attachment of dust to a product.

[0520] In particular, an insulating substrate is easily electrostatically
charged. An insulating substrate is formed using a material that is
easily electrostatically charged, such as glass or a resin.

[0521] Note that static electricity refers to charges in a state where,
when two objects are rubbed together, in contact with each other, or
separated from each other, one is positively charged and the other is
negatively charged. Charges are generated by movement of electrons
between two objects owing to friction or the like; such a phenomenon is
called electrification. When electrification is caused, generated charges
do not flow and are stored as static electricity in the case where a
material of an object is an insulator.

[0522] Moreover, a thin film transistor including an oxide semiconductor
layer has a possibility that electric characteristics of the thin film
transistor may fluctuate by the influence of static electricity and
deviate from the designed range.

[0523] Thus, after the first substrate provided with the thin film
transistor and the second substrate serving as the counter substrate are
bonded to each other, heat treatment is performed in a state where static
electricity stored in the thin film transistor is released to a ground
side and the charging amount of static electricity is gradually decreased
so that the static electricity is eliminated more easily. When this heat
treatment also serves as at least one of heat treatments performed in
manufacturing the display panel, the charging amount of static
electricity can be reduced without increase in the number of steps.

[0524] The case of manufacturing a liquid crystal display panel is
described below with reference to FIGS. 40A to 40C.

[0525] First, a first substrate 701 over which a thin film transistor 710
including an oxide semiconductor layer and a pixel electrode 730 are
formed in accordance with Embodiment 2 is prepared. Further, a driver
circuit is provided over the first substrate 701, and a thin film
transistor 711 in the driver circuit is manufactured through the same
process as the thin film transistor 710. The thin film transistor 711 is
covered with an interlayer insulating film 742, and the pixel electrode
730 is formed over the interlayer insulating film 742. A conductive layer
740 is formed above the thin film transistor 711 in the driver circuit to
block static electricity. Note that the conductive layer 740 is formed
using the same material as the pixel electrode 730.

[0526] After the pixel electrode is formed, cleaning is performed and then
drying is performed at 150° C. for 2 minutes. Next, an alignment
film is formed. The alignment film is formed in such a manner that a
liquid material for forming a horizontal alignment film (or a liquid
material for forming a vertical alignment film), such as polyimide, is
selectively applied by an offset printing method, a screen printing
method, or the like, and baked. Prebaking is performed with a hot plate
at 80° C. for 2 minutes and then baking is performed with a clean
oven at 230° C. for 40 minutes. After the baking, rubbing
treatment is performed. Then, cleaning is performed, and drying is
performed at 150° C. for 2 minutes.

[0527] A process of forming a color filter, an alignment film, a sealant,
and the like on a second substrate 706 serving as a counter substrate is
described below.

[0528] First, a black resin layer pattern serving as a black matrix is
formed on the second substrate 706. Next, a green resin layer pattern, a
blue resin layer pattern, and a red resin layer pattern are formed. The
green resin layer pattern, the blue resin layer pattern, and the red
resin layer pattern form the color filter. Then, an overcoat layer is
formed to cover these resin layer patterns.

[0529] Next, a counter electrode 731 including indium tin oxide to which
silicon oxide is added is formed on the overcoat layer by a sputtering
method. In order to reduce resistance of the counter electrode 731,
heating is performed at 250° C. for 1 hour.

[0530] Next, a columnar spacer 735 is formed on the counter electrode 731.
The columnar spacer 735 is obtained by selectively etching an organic
resin film such as an acrylic resin film.

[0531] Next, cleaning is performed, and drying is performed at 150°
C. for 2 minutes. Then, an alignment film is formed on the spacer 735.
The alignment film is formed in such a manner that a liquid material for
forming a horizontal alignment film (or a liquid material for forming a
vertical alignment film), such as polyimide, is selectively applied by an
offset printing method, a screen printing method, or the like, and baked.
Prebaking is performed with a hot plate at 80° C. for 2 minutes
and then baking is performed with a clean oven at 230° C. for 40
minutes. After the baking, rubbing treatment is performed. Then, cleaning
is performed, and drying is performed at 150° C. for 2 minutes.

[0532] Next, a sealant is formed by a screen printing method, or using an
inkjet apparatus or a dispensing apparatus. For the sealant, an
acrylic-based photocurable resin or the like may be used. As the sealant,
a sealant which includes a filler (with a diameter of 6 μm to 24
μm) and has a viscosity of 40 Pas to 400 Pas is used. Note that it is
preferable to select a sealant which is not dissolved in liquid crystal
with which the sealant is in contact later. This sealant is formed into a
closed loop and surrounds a display region.

[0533] In order to electrically connect the counter electrode 731 to a
common connection portion 702 provided over the first substrate, a
sealant 704 including conductive particles is also formed using an inkjet
apparatus or a dispensing apparatus. The common connection portion 702 is
provided in a position overlapping with the sealant for bonding the first
substrate and the second substrate and is electrically connected to the
counter electrode through the conductive particles in the sealant.
Alternatively, the common connection portion is provided in a position
that does not overlap with the sealant (except for the pixel portion) and
a paste including conductive particles is provided separately from the
sealant so as to overlap with the common connection portion, whereby the
common connection portion is electrically connected to the counter
electrode. The common connection portion 702 is formed using the same
material and through the same process as the pixel electrode 730 and the
conductive layer 740.

[0534] It does not matter if the second substrate 706 is electrostatically
charged until the formation of the sealant because an element such as a
thin film transistor is not formed yet; however, since the second
substrate 706 is bonded to the first substrate in a later step, it is
preferable that the charging amount of the second substrate 706 be
reduced before the bonding. In this case, the charging amount of the
second substrate 706 may be reduced with an ionizer or the like, or heat
treatment such as the above baking may be performed in a state where the
counter electrode 731 is electrically connected to a fixed potential, for
example, a ground potential.

[0535] Next, liquid crystal is dripped on the alignment film of the second
substrate 706. The dripping of a liquid crystal material is performed
using an inkjet apparatus or a dispensing apparatus under atmospheric
pressure. There is no particular limitation on the liquid crystal
material, and TN liquid crystal, OCB liquid crystal, STN liquid crystal,
VA liquid crystal, ECB liquid crystal, GH liquid crystal, polymer
dispersed liquid crystal, discotic liquid crystal, or the like can be
used.

[0536] Next, the pair of substrates is bonded to each other under reduced
pressure. The second substrate 706 where the liquid crystal is dripped is
bonded to the first substrate 701 provided with the thin film transistor
710. Immediately after bonding of the substrates, a sealant 705 is
irradiated with ultraviolet.

[0537] Next, in order to cure the sealant 705 further, heat treatment is
performed at higher than or equal to 80° C. and lower than or
equal to 200° C. for longer than or equal to 0.5 hour and shorter
than or equal to 10 hours. Note that in this heat treatment, the pair of
substrates bonded to each other is put in a furnace 780 of a heating
apparatus as illustrated in FIG. 40A. The furnace 780 is set over and in
contact with a stainless-steel floor which is electrically connected to a
ground potential so that the furnace 780 is electrically connected to the
ground potential. Then, heating is performed while an external terminal
716 which is connected to the ground potential is connected to a common
connection terminal 715 which is electrically connected to the common
connection portion 702. Alternatively, the furnace 780 and the common
connection portion 702 may be electrically connected to a fixed potential
without limitation to the ground potential (also referred to as GND).
Cure of the sealant 705 and appropriate removal of stored static
electricity can be performed at the same time by this heat treatment.

[0538] In this embodiment, heating is performed at 120° C. for 1
hour.

[0539] Note that FIG. 40B is an enlarged cross-sectional view of a display
region which is subjected to the heat treatment in a state of being
connected to the ground potential. As illustrated in FIG. 40B, a liquid
crystal layer 708 is provided between the counter electrode 731 which is
electrically connected to the ground potential and the pixel electrode
730 which is electrically connected to the thin film transistor 710. By
heating the liquid crystal layer 708, static electricity 790 stored in
the thin film transistor 710 is released to the ground side through the
liquid crystal layer 708. FIG. 40C is a schematic view illustrating the
above state simply. FIG. 40C, which is the schematic view using an
equivalent circuit, illustrates a path 791 through which the static
electricity 790 stored in the thin film transistor 710 is released to the
ground side through the liquid crystal layer. By the heat treatment,
stored static electricity is released to the ground side through the path
791 and gradually decreased to be eliminated easily.

[0540] By performing heat treatment with the counter electrode set at the
ground potential, a normally-off thin film transistor can be manufactured
stably; accordingly, yield of the liquid crystal display panel can be
improved.

[0541] In the case of manufacturing a plurality of panels from one
substrate, after bonding of the pair of substrates, the first substrate
or both substrates is/are cut using a cutting apparatus such as a scriber
apparatus, a breaker apparatus, or a roll cutter. Thus, a plurality of
panels can be manufactured from one substrate.

[0542] Next, heat treatment for aligning liquid crystal, that is,
realignment treatment is performed (e.g., at 80° C. to 200°
C. for 10 minutes to 1 hour, preferably at 100° C. to 170°
C. for 10 minutes to 1 hour).

[0543] In this embodiment, heating at 120° C. for 1 hour is
performed as the realignment treatment. This heat treatment may be
performed with the counter electrode set at the ground potential as
illustrated in FIG. 40A. Further, cure of the sealant and alignment of
the liquid crystal are performed by separate heat treatments as an
example in this embodiment, but may be performed by one heat treatment.

[0544] Through the above process, the liquid crystal display panel can be
formed.

[0545] In addition, without limitation to the liquid crystal display
device, the heat treatment for reducing stored static electricity can be
performed on a display panel such as electronic paper where electronic
ink is driven, which is described in Embodiment 8. For example, heat
treatment for curing a sealant with which a second substrate that is used
for sealing of the electronic ink is fixed to a first substrate provided
with a thin film transistor may be performed while an electrode provided
on the second substrate is electrically connected to a ground potential.
When heat treatment is performed with the electrode provided on the
second substrate set at the ground potential, a normally-off thin film
transistor can be manufactured stably; accordingly, yield of the active
matrix electronic paper can be improved.

[0546] Furthermore, without limitation to the liquid crystal display
device, the heat treatment for reducing stored static electricity can be
performed on the EL display panel described in Embodiment 9.

[0547] In the case of manufacturing an EL display panel, a first electrode
which is electrically connected to a thin film transistor including an
oxide semiconductor layer and a partition wall which covers a periphery
of the first electrode are formed over a first substrate in accordance
with Embodiment 2, and then heating is performed. This heating is
conducted in the following manner: heat treatment is performed at
200° C. for 1 hour in a nitrogen atmosphere and further at
150° C. for 1 hour in vacuum, and then a layer including an
organic compound is evaporated over the first electrode of the first
substrate.

[0548] Next, a second electrode is formed over the layer including an
organic compound by an evaporation method or a sputtering method. The
second electrode is provided above and to overlap with the thin film
transistor in a display region. Further, the second electrode can also be
provided above and to overlap with a thin film transistor in a driver
circuit. When the second electrode is at a common potential, it is
preferable that the second electrode be electrically connected to a
ground potential during heat treatment performed later.

[0549] Next, a second substrate having a depression where a drying agent
is set is fixed to the first substrate with a sealant, and heat treatment
for curing the sealant is performed. In the case of an EL display panel,
a light-emitting element might deteriorate at a heating temperature
higher than 80° C.; therefore, the heat treatment is performed at
80° C. for longer than or equal to 0.5 hour and shorter than or
equal to 10 hours.

[0550] By performing heat treatment with the second electrode set at the
ground potential, a normally-off thin film transistor can be manufactured
stably; accordingly, yield of the EL display panel can be improved.

[0551] In the case where sealing of the light-emitting element is
performed using a stainless-steel substrate with a small thickness as the
second substrate, heat treatment is performed at the time of curing an
adhesive (such as an epoxy resin) for fixing the stainless-steel
substrate while the stainless-steel substrate is electrically connected
to the ground potential. In the case of using the stainless-steel
substrate, the stainless-steel substrate including a conductive material
overlaps with all thin film transistors including the thin film
transistor in the display region and the thin film transistor in the
driver circuit which are formed over one substrate. By performing heat
treatment with the stainless-steel substrate overlapping with the thin
film transistors set at a fixed potential such as the ground potential,
normally-off thin film transistors can be manufactured stably;
accordingly, yield of the EL display panel which is flexible can be
improved.

[0552] By performing heat treatment with an electrode overlapping with a
thin film transistor set at a fixed potential such as a ground potential,
static electricity stored in a substrate in a manufacturing process of a
semiconductor device can be favorably removed.

Embodiment 14

[0553] In this embodiment, as for the channel-etched thin film transistor
in which an In--Ga--Zn--O-based oxide semiconductor film is used as an
active layer, which is described in Embodiment 2, the following
phenomenon was examined by computational science: a layer including
indium at a higher concentration than the other region (In-rich layer)
and a titanium oxide (TiOx) film are formed in the vicinity of an
interface between the In--Ga--Zn--O-based oxide semiconductor film and a
metal film used for a source electrode or a drain electrode.

[0554] First, energy which is necessary for an oxide of each of indium,
gallium, and zinc that are included in the In--Ga--Zn--O-based oxide
semiconductor to form an oxygen-deficient state (deficiency formation
energy Edef) was calculated, and which metal oxide was likely to
form an oxygen-deficient state was studied.

[0555] Note that the deficiency formation energy Edef is expressed by
Formula 1 below. A represents any of indium; gallium; zinc; and a
combination of indium, gallium, and zinc. Note that E(O) represents half
energy of an oxygen atom, and E(AmOn-1) represents energy of an
oxide AmOn-1 with oxygen deficiency.

Edef=(E(AmOn-1)+E(O))-E(AmOn) (Formula 1)

[0556] An approximate relation between a concentration of deficiency n and
the deficiency formation energy Edef is expressed by Formula 2
below. Note that N represents the number of oxygen positions in a state
where deficiency is not formed, kB represents Boltzmann constant,
and T represents temperature.

n=N×exp(-Edef/kBT) (Formula 2)

[0557] The calculation was performed using CASTEP, which is a program
using the density functional theory. A plane-wave-basis pseudopotential
method was used as the density functional theory, and GGA-PBE was used
for a functional. The cut-off energy was 500 eV. The number of grids at
k-point was set as follows: 3×3×1 for IGZO; 2×2×2
for In2O3; 2×3×2 for Ga2O3; and
4×4×1 for ZnO.

[0558] As a crystal structure of IGZO crystal, a structure, in which a
structure where 84 atoms obtained by doubling the a-axis and the b-axis
in a symmetry R-3 (international number: 148) structure were arranged so
that energy of Ga and Zn was minimized, was employed. As for
In2O3, a bixbyite structure of 80 atoms was employed; as for
Ga2O3, a β-Gallia structure of 80 atoms was employed; and
as for ZnO, a wurtzite structure of 80 atoms was employed.

[0559] From the Formula 2, it is found that when the deficiency formation
energy Edef is increased, the concentration of oxygen deficiency n,
that is, the amount of oxygen deficiency is decreased. Table 1 below
shows values of the deficiency formation energy Edef in the case
where A is indium; in the case where A is gallium; in the case where A is
zinc; and in the case where A is a combination of indium, gallium, and
zinc.

[0560] IGZO (Model 1) shows a value of the deficiency formation energy
Edef of oxygen that is adjacent to three indium atoms and one zinc
atom in a crystal. This structure is illustrated in FIG. 41A.

[0561] IGZO (Model 2) shows a value of the deficiency formation energy
Edef of oxygen that is adjacent to three indium atoms and one
gallium atom in a crystal. This structure is illustrated in FIG. 41B.

[0562] IGZO (Model 3) shows a value of the deficiency formation energy
Edef of oxygen that is adjacent to two zinc atoms and two gallium
atoms in a crystal. This structure is illustrated in FIG. 41c.

[0563] The larger the value of the deficiency formation energy Edef
is, the more energy is needed to form an oxygen-deficient state; that is,
this indicates bonding with oxygen tends to be strong. Accordingly, the
values of the deficiency formation energy Edef shown in Table 1
indicate that indium has the weakest bonding with oxygen and oxygen is
easily released in the vicinity of indium.

[0564] It is considered that an oxygen-deficient state in an
In--Ga--Zn--O-based oxide semiconductor is formed by extraction of oxygen
from the oxide semiconductor by metal used for a source electrode or a
drain electrode. Since the electric conductivity of an oxide
semiconductor is increased by formation of an oxygen-deficient state, the
electric conductivity of the oxide semiconductor film in the vicinity of
an interface with the metal film is expected to be increased by the
extraction of oxygen.

[0565] Next, in order to confirm whether oxygen is extracted from the
oxide semiconductor by metal, quantum molecular dynamic (QMD) simulation
was performed on a stacked-layer structure of the In--Ga--Zn--O-based
oxide semiconductor film and the metal film.

[0566] The structure used for the simulation was formed in the following
manner. First, structural optimization using first principle calculation
was performed on an amorphous In--Ga--Zn--O-based oxide semiconductor
(In:Ga:Zn:O=1:1:1:4, 84 atoms in total) formed by a classical molecular
dynamic (CMD) method. Crystal of metal atoms (W, Mo, and Ti) was stacked
over an a-IGZO layer obtained by cutting a unit lattice subjected to the
structural optimization, and structural optimization was performed.
Quantum molecular dynamic (QMD) simulation was performed using this
structure as an initial structure at 623.0 K. Note that a lower end of
the a-IGZO layer and an upper end of the metal layer were fixed so that
only interaction in the interface could be estimated.

[0567] The conditions of the classical molecular dynamic simulation are
shown below. Materials Explorer was used as a calculation program. The
a-IGZO was formed under the following conditions. The all 84 atoms were
arranged at random in a simulation cell with a side of 1 nm at a ratio of
1n:Ga:Zn:O=1:1:1:4, and the density was set to 5.9 g/cm3. The
temperature was gradually lowered from 5500 K to 1 K with an NVT
ensemble, and then structural relaxation was performed at 1 K for 10 ns.
The time interval was 0.1 fs, and the total calculation time was 10 ns.
As for potentials, a Born-Mayer-Huggins potential was applied to between
metal-oxygen and oxygen-oxygen, and a Lennard-Jones potential was applied
to between metal-metal. Charges were set as follows: +3 for In, +3 for
Ga, +2 for Zn, and -2 for 0.

[0568] The conditions of the QMD simulation are shown below. A
first-principle calculation software CASTEP was used as a calculation
program. GGA-PBE was used for a functional, and Ultrasoft was used for
pseudopotential. The cut-off energy was 260 eV, and the k-point set was
1×1×1. The MD simulation was performed using an NVT ensemble
at a temperature of 623 K. The total simulation time was 2.0 ps and the
time interval was 1.0 fs.

[0569] FIGS. 42A and 42B, FIGS. 43A and 43B, and FIGS. 44A and 44B show
results of the above simulation. In FIGS. 42A and 42B, FIGS. 43A and 43B,
and FIGS. 44A and 44B, a white sphere represents a metal atom, and a
black sphere represents an oxygen atom. FIGS. 42A and 42B show structures
in the case of using a metal layer including W. FIG. 42A shows a
structure before the QMD simulation, and FIG. 42B shows structure after
the QMD simulation. FIGS. 43A and 43B show structures in the case of
using a metal layer including Mo. FIG. 43A shows a structure before the
QMD simulation, and FIG. 43B shows structure after the QMD simulation.
FIGS. 44A and 44B show structures in the case of using a metal layer
including Ti. FIG. 44A shows a structure before the QMD simulation, and
FIG. 44B shows structure after the QMD simulation.

[0570] From FIG. 43A and FIG. 44A, in the cases of Mo and Ti, oxygen moved
into the metal layers is already observed at the time of structural
optimization. By comparing FIG. 42B, FIG. 43B, and FIG. 44B, it is found
that oxygen moves most frequently in the case of Ti. Accordingly, Ti is
considered to be suitable for an electrode that causes oxygen deficiency
in a-IGZO.

[0571] It is assumed that oxygen extracted by titanium reacts with
titanium and thus titanium oxide is formed. Therefore, whether a titanium
oxide film formed between an oxide semiconductor film and a titanium film
has conductivity was examined.

[0572] Titanium dioxide has several crystal structures such as a rutile
structure (high temperature tetragonal crystal), an anatase structure
(low temperature tetragonal crystal), and a brookite structure
(orthorhombic crystal). Since both the anatase structure and the brookite
structure are changed into the rutile structure by heating, which is the
most stable structure, the above titanium dioxide was assumed to have a
rutile structure. FIG. 45 shows a crystal structure of titanium dioxide
having a rutile structure. The rutile structure is tetragonal crystal and
the space group that represents symmetry of crystal is P42/mnm.

[0573] Simulation for obtaining density of states was performed on the
above structure of titanium dioxide by a density functional theory using
a GGA-PBE functional. The structure of titanium dioxide including a cell
structure was optimized with symmetry maintained, and the density of
states was calculated. A plane-wave pseudopotential method introduced
into a CASTEP code was employed for the density functional simulation.
The cut-off energy was 380 eV.

[0574]FIG. 46 shows the density of states of titanium dioxide having a
rutile structure. As shown in FIG. 46, because titanium dioxide having a
rutile structure has a band gap titanium dioxide has a density of states
like that of an insulator or a semiconductor. Note that a narrower band
gap tends to be estimated by the density functional theory; therefore,
the actual band gap of titanium dioxide is approximately 3.0 eV, which is
wider than the band gap in FIG. 46 showing the density of states.

[0575]FIG. 47 shows the density of states of titanium dioxide having a
rutile structure in the case of including oxygen deficiency.
Specifically, titanium oxide including 24 Ti atoms and 47 O atoms, which
was obtained by removing one O atom from titanium oxide including 24 Ti
atoms and 48 O atoms, was used for the simulation as a model. The density
of states shown in FIG. 47 indicates that the Fermi level is moved inside
the conduction band, which is like that of metal, and that titanium
dioxide has n-type conductivity in the case of including oxygen
deficiency.

[0576] FIG. 48 shows the density of states of titanium monoxide (TiO).
From FIG. 48, it is found that titanium monoxide has a density of states
like that of metal.

[0577] Therefore, from the density of states of titanium dioxide shown in
FIG. 46, the density of states of titanium dioxide including oxygen
deficiency shown in FIG. 47, and the density of states of titanium
monoxide shown in FIG. 48, the following assumption can be made: titanium
dioxide including oxygen deficiency (TiO2-δ) has n-type
conductivity in the range of 0<δ<1. Accordingly, it is
considered that, when titanium monoxide or titanium dioxide including
oxygen deficiency is included in the composition of a titanium oxide
film, the titanium oxide film is less likely to inhibit current flow
between an In--Ga--Zn--O-based oxide semiconductor film and a titanium
film.

[0578]FIG. 49 is a diagram showing an energy band between a source
electrode and a drain electrode of a thin film transistor. Note that FIG.
49 is a diagram in the case of a thin film transistor where an
In--Ga--Zn--O-based (IGZO) film is used as an oxide semiconductor film
and TiOx films are provided between the oxide semiconductor film and
the source electrode and between the oxide semiconductor film and the
drain electrode. Note that the thickness of each of the TiOx films
is greater than or equal to 0.1 nm and less than or equal to 10 nm. In
addition, the above oxide semiconductor film includes much metal (such as
In, Ga, and Zn) and is provided with a pair of composite layers which is
in contact with the pair of TiOx films. The electron affinity of the
In--Ga--Zn--O-based (IGZO) film in a region other than the composite
layers is 4.3 eV, that of the TiOx film is 4.3 eV, that of Ti as the
source electrode or the drain electrode is 4.1 eV, and that of the
composite layer is 4.5 eV. Note that in FIG. 49, the position of the band
of each substance is changed so that the positions of the Fermi levels
are common. When gate voltage is not applied, the Fermi level in IGZO is
in the vicinity of the center of the band gap because IGZO has a small
number of carriers, whereas the Fermi levels in the TiOx film and
the composite layer are positioned in the vicinity of the conduction band
because of having a large number of carriers. Therefore, in FIG. 49, the
value at the position of the conduction band of each substance is
different from the above relative value of the electron affinity. As
shown in FIG. 49, the composite layer has little variation in the
electron affinity; therefore, favorable connection structures can be
realized between the oxide semiconductor film and the source electrode
and between the oxide semiconductor film and the drain electrode.

[0579] This application is based on Japanese Patent Application serial no.
2009-235792 filed with Japan Patent Office on Oct. 9, 2009, the entire
contents of which are hereby incorporated by reference.